Refactor _subscribers into a singly linked list
Refactor _subscribers into a singly linked list

This reduces memory usage for the overwhelmingly common case of 1
subscriber, while still optimizing searching and insertion for other
cases.

zhukaixy/ReactiveCocoa,Ray0218/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,fanghao085/ReactiveCocoa,jaylib/ReactiveCocoa,dachaoisme/ReactiveCocoa,mxxiv/ReactiveCocoa,Farteen/ReactiveCocoa,j364960953/ReactiveCocoa,nikita-leonov/ReactiveCocoa,cstars135/ReactiveCocoa,OneSmallTree/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,clg0118/ReactiveCocoa,lixar/ReactiveCocoa,LHDsimon/ReactiveCocoa,itschaitanya/ReactiveCocoa,OneSmallTree/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,Pingco/ReactiveCocoa,CQXfly/ReactiveCocoa,xulibao/ReactiveCocoa,jam891/ReactiveCocoa,zzzworm/ReactiveCocoa,monkeydbobo/ReactiveCocoa,dullgrass/ReactiveCocoa,imkerberos/ReactiveCocoa,kiurentu/ReactiveCocoa,llb1119/test,Pikdays/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,zhiwen1024/ReactiveCocoa,DreamHill/ReactiveCocoa,bscarano/ReactiveCocoa,howandhao/ReactiveCocoa,Ricowere/ReactiveCocoa,zxq3220122/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,ShawnLeee/ReactiveCocoa,AndyZhaoHe/ReactiveCocoa,nikita-leonov/ReactiveCocoa,SanChain/ReactiveCocoa,200895045/ReactiveCocoa,eyu1988/ReactiveCocoa,brightcove/ReactiveCocoa,wpstarnice/ReactiveCocoa,zhigang1992/ReactiveCocoa,andersio/ReactiveCocoa,yonekawa/ReactiveCocoa,jeelun/ReactiveCocoa,tzongw/ReactiveCocoa,andersio/ReactiveCocoa,200895045/ReactiveCocoa,zzqiltw/ReactiveCocoa,zxq3220122/ReactiveCocoa,335g/ReactiveCocoa,wangqi211/ReactiveCocoa,JackLian/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,kiurentu/ReactiveCocoa,xulibao/ReactiveCocoa,sujeking/ReactiveCocoa,eliperkins/ReactiveCocoa,Carthage/ReactiveCocoa,tonyarnold/ReactiveCocoa,zhukaixy/ReactiveCocoa,monkeydbobo/ReactiveCocoa,qq644531343/ReactiveCocoa,gabemdev/ReactiveCocoa,bensonday/ReactiveCocoa,shuxiashusheng/ReactiveCocoa,rpowelll/ReactiveCocoa,200895045/ReactiveCocoa,Liquidsoul/ReactiveCocoa,brightcove/ReactiveCocoa,yonekawa/ReactiveCocoa,zhigang1992/ReactiveCocoa,smilypeda/ReactiveCocoa,xumaolin/ReactiveCocoa,paulyoung/ReactiveCocoa,Ethan89/ReactiveCocoa,libiao88/ReactiveCocoa,sugar2010/ReactiveCocoa,on99/ReactiveCocoa,bensonday/ReactiveCocoa,ohwutup/ReactiveCocoa,ztchena/ReactiveCocoa,chao95957/ReactiveCocoa,tonyli508/ReactiveCocoa,emodeqidao/ReactiveCocoa,yytong/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,hoanganh6491/ReactiveCocoa,ericzhou2008/ReactiveCocoa,calebd/ReactiveCocoa,Rupert-RR/ReactiveCocoa,cnbin/ReactiveCocoa,leichunfeng/ReactiveCocoa,stupidfive/ReactiveCocoa,wpstarnice/ReactiveCocoa,mxxiv/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,ioshger0125/ReactiveCocoa,Remitly/ReactiveCocoa,eyu1988/ReactiveCocoa,ioshger0125/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,liufeigit/ReactiveCocoa,ceekayel/ReactiveCocoa,fhchina/ReactiveCocoa,chao95957/ReactiveCocoa,dullgrass/ReactiveCocoa,juliangrosshauser/ReactiveCocoa,zzzworm/ReactiveCocoa,jrmiddle/ReactiveCocoa,imkerberos/ReactiveCocoa,Pikdays/ReactiveCocoa,esttorhe/ReactiveCocoa,dachaoisme/ReactiveCocoa,tonyli508/ReactiveCocoa,Ricowere/ReactiveCocoa,sdhzwm/ReactiveCocoa,nickcheng/ReactiveCocoa,JohnJin007/ReactiveCocoa,yizzuide/ReactiveCocoa,almassapargali/ReactiveCocoa,FelixYin66/ReactiveCocoa,howandhao/ReactiveCocoa,BlessNeo/ReactiveCocoa,mtxs007/ReactiveCocoa,Khan/ReactiveCocoa,koamac/ReactiveCocoa,ailyanlu/ReactiveCocoa,walkingsmarts/ReactiveCocoa,j364960953/ReactiveCocoa,brightcove/ReactiveCocoa,fanghao085/ReactiveCocoa,jsslai/ReactiveCocoa,bscarano/ReactiveCocoa,natan/ReactiveCocoa,BrooksWon/ReactiveCocoa,emodeqidao/ReactiveCocoa,Eveian/ReactiveCocoa,almassapargali/ReactiveCocoa,tornade0913/ReactiveCocoa,huiping192/ReactiveCocoa,rpowelll/ReactiveCocoa,BrooksWon/ReactiveCocoa,llb1119/test,jrmiddle/ReactiveCocoa,Carthage/ReactiveCocoa,kevin-zqw/ReactiveCocoa,Remitly/ReactiveCocoa,FelixYin66/ReactiveCocoa,walkingsmarts/ReactiveCocoa,esttorhe/ReactiveCocoa,chieryw/ReactiveCocoa,sdhzwm/ReactiveCocoa,shaohung001/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,mattpetters/ReactiveCocoa,JackLian/ReactiveCocoa,yoichitgy/ReactiveCocoa,jackywpy/ReactiveCocoa,sugar2010/ReactiveCocoa,zhigang1992/ReactiveCocoa,stupidfive/ReactiveCocoa,huiping192/ReactiveCocoa,zhaoguohui/ReactiveCocoa,Ricowere/ReactiveCocoa,hilllinux/ReactiveCocoa,yizzuide/ReactiveCocoa,tornade0913/ReactiveCocoa,chieryw/ReactiveCocoa,esttorhe/ReactiveCocoa,KJin99/ReactiveCocoa,zhiwen1024/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,taylormoonxu/ReactiveCocoa,LHDsimon/ReactiveCocoa,Juraldinio/ReactiveCocoa,hbucius/ReactiveCocoa,xiaoliyang/ReactiveCocoa,yaoxiaoyong/ReactiveCocoa,on99/ReactiveCocoa,buildo/ReactiveCocoa,smilypeda/ReactiveCocoa,alvinvarghese/ReactiveCocoa,Ethan89/ReactiveCocoa,brasbug/ReactiveCocoa,OneSmallTree/ReactiveCocoa,brasbug/ReactiveCocoa,sandyway/ReactiveCocoa,itschaitanya/ReactiveCocoa,ohwutup/ReactiveCocoa,hj3938/ReactiveCocoa,victorlin/ReactiveCocoa,ericzhou2008/ReactiveCocoa,tiger8888/ReactiveCocoa,ioshger0125/ReactiveCocoa,sdhzwm/ReactiveCocoa,hilllinux/ReactiveCocoa,luerhouhou/ReactiveCocoa,tonyarnold/ReactiveCocoa,ShawnLeee/ReactiveCocoa,yizzuide/ReactiveCocoa,zzqiltw/ReactiveCocoa,valleyman86/ReactiveCocoa,dz1111/ReactiveCocoa,eliperkins/ReactiveCocoa,natestedman/ReactiveCocoa,hoanganh6491/ReactiveCocoa,yonekawa/ReactiveCocoa,kevin-zqw/ReactiveCocoa,zhiwen1024/ReactiveCocoa,lixar/ReactiveCocoa,mtxs007/ReactiveCocoa,libiao88/ReactiveCocoa,jeelun/ReactiveCocoa,yytong/ReactiveCocoa,terry408911/ReactiveCocoa,Adlai-Holler/ReactiveCocoa,BrooksWon/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,bensonday/ReactiveCocoa,tzongw/ReactiveCocoa,llb1119/test,vincentiss/ReactiveCocoa,koamac/ReactiveCocoa,bencochran/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,leelili/ReactiveCocoa,takeshineshiro/ReactiveCocoa,eliperkins/ReactiveCocoa,xiaobing2007/ReactiveCocoa,zhenlove/ReactiveCocoa,icepy/ReactiveCocoa,dskatz22/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,beni55/ReactiveCocoa,AndyZhaoHe/ReactiveCocoa,hoanganh6491/ReactiveCocoa,kaylio/ReactiveCocoa,kaylio/ReactiveCocoa,JohnJin007/ReactiveCocoa,windgo/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,jianwoo/ReactiveCocoa,FelixYin66/ReactiveCocoa,dachaoisme/ReactiveCocoa,clg0118/ReactiveCocoa,longv2go/ReactiveCocoa,KuPai32G/ReactiveCocoa,zxq3220122/ReactiveCocoa,jianwoo/ReactiveCocoa,huiping192/ReactiveCocoa,towik/ReactiveCocoa,liufeigit/ReactiveCocoa,Eveian/ReactiveCocoa,leelili/ReactiveCocoa,goodheart/ReactiveCocoa,KuPai32G/ReactiveCocoa,AlanJN/ReactiveCocoa,yoichitgy/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,bscarano/ReactiveCocoa,luerhouhou/ReactiveCocoa,dz1111/ReactiveCocoa,richeterre/ReactiveCocoa,jeelun/ReactiveCocoa,Adlai-Holler/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,rpowelll/ReactiveCocoa,loupman/ReactiveCocoa,SmartEncounter/ReactiveCocoa,beni55/ReactiveCocoa,leichunfeng/ReactiveCocoa,msdgwzhy6/ReactiveCocoa,emodeqidao/ReactiveCocoa,Khan/ReactiveCocoa,chieryw/ReactiveCocoa,BlessNeo/ReactiveCocoa,ikesyo/ReactiveCocoa,takeshineshiro/ReactiveCocoa,Liquidsoul/ReactiveCocoa,tornade0913/ReactiveCocoa,xumaolin/ReactiveCocoa,AlanJN/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,tzongw/ReactiveCocoa,gengjf/ReactiveCocoa,jaylib/ReactiveCocoa,terry408911/ReactiveCocoa,hbucius/ReactiveCocoa,ddc391565320/ReactiveCocoa,howandhao/ReactiveCocoa,Pingco/ReactiveCocoa,qq644531343/ReactiveCocoa,windgo/ReactiveCocoa,isghe/ReactiveCocoa,leelili/ReactiveCocoa,mattpetters/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,zhaoguohui/ReactiveCocoa,bencochran/ReactiveCocoa,richeterre/ReactiveCocoa,nickcheng/ReactiveCocoa,luerhouhou/ReactiveCocoa,zzqiltw/ReactiveCocoa,isghe/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,Carthage/ReactiveCocoa,Adlai-Holler/ReactiveCocoa,tonyarnold/ReactiveCocoa,tipbit/ReactiveCocoa,isghe/ReactiveCocoa,longv2go/ReactiveCocoa,CCOOOOLL/ReactiveCocoa,cogddo/ReactiveCocoa,WEIBP/ReactiveCocoa,ddc391565320/ReactiveCocoa,CQXfly/ReactiveCocoa,tiger8888/ReactiveCocoa,JohnJin007/ReactiveCocoa,paulyoung/ReactiveCocoa,DreamHill/ReactiveCocoa,KJin99/ReactiveCocoa,walkingsmarts/ReactiveCocoa,WEIBP/ReactiveCocoa,cstars135/ReactiveCocoa,KuPai32G/ReactiveCocoa,towik/ReactiveCocoa,juliangrosshauser/ReactiveCocoa,ericzhou2008/ReactiveCocoa,jaylib/ReactiveCocoa,beni55/ReactiveCocoa,valleyman86/ReactiveCocoa,leichunfeng/ReactiveCocoa,monkeydbobo/ReactiveCocoa,sujeking/ReactiveCocoa,xiaoliyang/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,sandyway/ReactiveCocoa,stevielu/ReactiveCocoa,jaylib/ReactiveCocoa,hj3938/ReactiveCocoa,OnTheWay1988/ReactiveCocoa,pzw224/ReactiveCocoa,nickcheng/ReactiveCocoa,shuxiashusheng/ReactiveCocoa,goodheart/ReactiveCocoa,buildo/ReactiveCocoa,on99/ReactiveCocoa,SanChain/ReactiveCocoa,dskatz22/ReactiveCocoa,icepy/ReactiveCocoa,bencochran/ReactiveCocoa,Juraldinio/ReactiveCocoa,natan/ReactiveCocoa,zhenlove/ReactiveCocoa,SuPair/ReactiveCocoa,mxxiv/ReactiveCocoa,alvinvarghese/ReactiveCocoa,dz1111/ReactiveCocoa,vincentiss/ReactiveCocoa,ailyanlu/ReactiveCocoa,longv2go/ReactiveCocoa,icepy/ReactiveCocoa,koamac/ReactiveCocoa,xulibao/ReactiveCocoa,mattpetters/ReactiveCocoa,shaohung001/ReactiveCocoa,esttorhe/ReactiveCocoa,fanghao085/ReactiveCocoa,cnbin/ReactiveCocoa,terry408911/ReactiveCocoa,SmartEncounter/ReactiveCocoa,huiping192/ReactiveCocoa,stevielu/ReactiveCocoa,fhchina/ReactiveCocoa,hj3938/ReactiveCocoa,Eveian/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,taylormoonxu/ReactiveCocoa,yangshengchaoios/ReactiveCocoa,mtxs007/ReactiveCocoa,calebd/ReactiveCocoa,ohwutup/ReactiveCocoa,qq644531343/ReactiveCocoa,LHDsimon/ReactiveCocoa,SuPair/ReactiveCocoa,goodheart/ReactiveCocoa,Ray0218/ReactiveCocoa,CQXfly/ReactiveCocoa,GuitarPlayer-Ma/ReactiveCocoa,ailyanlu/ReactiveCocoa,Pingco/ReactiveCocoa,kevin-zqw/ReactiveCocoa,zhaoguohui/ReactiveCocoa,Juraldinio/ReactiveCocoa,zzzworm/ReactiveCocoa,hbucius/ReactiveCocoa,BlessNeo/ReactiveCocoa,cogddo/ReactiveCocoa,cnbin/ReactiveCocoa,ikesyo/ReactiveCocoa,tiger8888/ReactiveCocoa,KJin99/ReactiveCocoa,gengjf/ReactiveCocoa,kiurentu/ReactiveCocoa,shaohung001/ReactiveCocoa,yytong/ReactiveCocoa,alvinvarghese/ReactiveCocoa,fhchina/ReactiveCocoa,kaylio/ReactiveCocoa,windgo/ReactiveCocoa,WEIBP/ReactiveCocoa,gabemdev/ReactiveCocoa,victorlin/ReactiveCocoa,Ricowere/ReactiveCocoa,ceekayel/ReactiveCocoa,add715/ReactiveCocoa,eyu1988/ReactiveCocoa,tonyli508/ReactiveCocoa,juliangrosshauser/ReactiveCocoa,nikita-leonov/ReactiveCocoa,DreamHill/ReactiveCocoa,jam891/ReactiveCocoa,sugar2010/ReactiveCocoa,imkerberos/ReactiveCocoa,pzw224/ReactiveCocoa,takeshineshiro/ReactiveCocoa,wpstarnice/ReactiveCocoa,yoichitgy/ReactiveCocoa,SuPair/ReactiveCocoa,Rupert-RR/ReactiveCocoa,victorlin/ReactiveCocoa,hilllinux/ReactiveCocoa,ztchena/ReactiveCocoa,smilypeda/ReactiveCocoa,AlanJN/ReactiveCocoa,xiaoliyang/ReactiveCocoa,shuxiashusheng/ReactiveCocoa,dskatz22/ReactiveCocoa,lixar/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,almassapargali/ReactiveCocoa,yangyangluoluo/ReactiveCocoa,loupman/ReactiveCocoa,natestedman/ReactiveCocoa,zhenlove/ReactiveCocoa,AndyZhaoHe/ReactiveCocoa,wangqi211/ReactiveCocoa,DongDongDongDong/ReactiveCocoa,335g/ReactiveCocoa,Rupert-RR/ReactiveCocoa,xumaolin/ReactiveCocoa,sandyway/ReactiveCocoa,Farteen/ReactiveCocoa,towik/ReactiveCocoa,nickcheng/ReactiveCocoa,calebd/ReactiveCocoa,jsslai/ReactiveCocoa,stevielu/ReactiveCocoa,ShawnLeee/ReactiveCocoa,JackLian/ReactiveCocoa,itschaitanya/ReactiveCocoa,AllanChen/ReactiveCocoa,j364960953/ReactiveCocoa,stupidfive/ReactiveCocoa,natan/ReactiveCocoa,clg0118/ReactiveCocoa,tipbit/ReactiveCocoa,Ray0218/ReactiveCocoa,dullgrass/ReactiveCocoa,jam891/ReactiveCocoa,Farteen/ReactiveCocoa,buildo/ReactiveCocoa,AllanChen/ReactiveCocoa,natestedman/ReactiveCocoa,Khan/ReactiveCocoa,wangqi211/ReactiveCocoa,ztchena/ReactiveCocoa,sujeking/ReactiveCocoa,jackywpy/ReactiveCocoa,chao95957/ReactiveCocoa,jrmiddle/ReactiveCocoa,Ethan89/ReactiveCocoa,jianwoo/ReactiveCocoa,xiaobing2007/ReactiveCocoa,cogddo/ReactiveCocoa,zhukaixy/ReactiveCocoa,vincentiss/ReactiveCocoa,SanChain/ReactiveCocoa,pzw224/ReactiveCocoa,richeterre/ReactiveCocoa,Liquidsoul/ReactiveCocoa,ceekayel/ReactiveCocoa,taylormoonxu/ReactiveCocoa,andersio/ReactiveCocoa,jackywpy/ReactiveCocoa,335g/ReactiveCocoa,brasbug/ReactiveCocoa,ddc391565320/ReactiveCocoa,PSPDFKit-labs/ReactiveCocoa,loupman/ReactiveCocoa,libiao88/ReactiveCocoa,gengjf/ReactiveCocoa,paulyoung/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,cstars135/ReactiveCocoa,add715/ReactiveCocoa,liufeigit/ReactiveCocoa,valleyman86/ReactiveCocoa,Pikdays/ReactiveCocoa,xiaobing2007/ReactiveCocoa,add715/ReactiveCocoa,jsslai/ReactiveCocoa,Remitly/ReactiveCocoa