adding StringPiece constructor that takes a piece of another StringPiece
adding StringPiece constructor that takes a piece of another StringPiece

Summary:
adding what seems to be a missing constructor to StringPiece --
I need to be able to take a piece of another StringPiece. (It's possible
to do that with all sorts of strings already, except StringPiece...)

Test Plan: na -- tested as part of other (dependent) diffs

Reviewed By: delong.j@fb.com

FB internal diff: D508545

bsampath/folly,Orvid/folly,hongliangzhao/folly,floxard/folly,brunomorishita/folly,gaoyingie/folly,charsyam/folly,fw1121/folly,clearlinux/folly,floxard/folly,bobegir/folly,leolujuyi/folly,mqeizi/folly,brunomorishita/folly,kernelim/folly,hongliangzhao/folly,renyinew/folly,CJstar/folly,KSreeHarsha/folly,loverszhaokai/folly,sonnyhu/folly,nickhen/folly,guker/folly,guker/folly,loversInJapan/folly,lifei/folly,wildinto/folly,SeanRBurton/folly,nickhen/folly,nickhen/folly,chjp2046/folly,upsoft/folly,alexst07/folly,rklabs/folly,cppfool/folly,nickhen/folly,leolujuyi/folly,loversInJapan/folly,tomhughes/folly,SammyK/folly,PoisonBOx/folly,shaobz/folly,PoisonBOx/folly,Orvid/folly,brunomorishita/folly,unmeshvrije/C--,constantine001/folly,sakishum/folly,hongliangzhao/folly,shaobz/folly,theiver9827/folly,leolujuyi/folly,PPC64/folly,gaoyingie/folly,KSreeHarsha/folly,colemancda/folly,hongliangzhao/folly,yangjin-unique/folly,zhiweicai/folly,zhiweicai/folly,clearlylin/folly,alexst07/folly,clearlylin/folly,raphaelamorim/folly,Orvid/folly,yangjin-unique/folly,bikong2/folly,Eagle-X/folly,cole14/folly,romange/folly,PoisonBOx/folly,upsoft/folly,hongliangzhao/folly,constantine001/folly,PPC64/folly,lifei/folly,stonegithubs/folly,raphaelamorim/folly,yangjin-unique/folly,clearlinux/folly,clearlylin/folly,chjp2046/folly,bowlofstew/folly,sonnyhu/folly,colemancda/folly,bsampath/folly,Hincoin/folly,juniway/folly,renyinew/folly,cppfool/folly,Orvid/folly,tempbottle/folly,colemancda/folly,xzmagic/folly,Eagle-X/folly,alexst07/folly,colemancda/folly,theiver9827/folly,SeanRBurton/folly,loverszhaokai/folly,raphaelamorim/folly,arg0/folly,SammyK/folly,kernelim/folly,cole14/folly,mqeizi/folly,cole14/folly,tomhughes/folly,arg0/folly,brunomorishita/folly,bsampath/folly,facebook/folly,KSreeHarsha/folly,ajtulloch/folly,raphaelamorim/folly,upsoft/folly,juniway/folly,bsampath/folly,shaobz/folly,sakishum/folly,xzmagic/folly,bobegir/folly,reddit/folly,reddit/folly,tomhughes/folly,wildinto/folly,fw1121/folly,upsoft/folly,rklabs/folly,loversInJapan/folly,renyinew/folly,rklabs/folly,CJstar/folly,PPC64/folly,reddit/folly,KSreeHarsha/folly,cole14/folly,fw1121/folly,ykarlsbrun/folly,facebook/folly,ykarlsbrun/folly,zhiweicai/folly,rklabs/folly,floxard/folly,cppfool/folly,sakishum/folly,project-zerus/folly,Hincoin/folly,clearlinux/folly,unmeshvrije/C--,charsyam/folly,tempbottle/folly,SammyK/folly,mqeizi/folly,charsyam/folly,alexst07/folly,gavioto/folly,bowlofstew/folly,bowlofstew/folly,arg0/folly,upsoft/folly,guker/folly,bobegir/folly,bikong2/folly,floxard/folly,fw1121/folly,SeanRBurton/folly,shaobz/folly,Eagle-X/folly,kernelim/folly,SammyK/folly,bsampath/folly,theiver9827/folly,facebook/folly,unmeshvrije/C--,bowlofstew/folly,loverszhaokai/folly,floxard/folly,project-zerus/folly,theiver9827/folly,CJstar/folly,xzmagic/folly,xzmagic/folly,Hincoin/folly,tempbottle/folly,yangjin-unique/folly,lifei/folly,lifei/folly,alexst07/folly,project-zerus/folly,KSreeHarsha/folly,doctaweeks/folly,mqeizi/folly,guker/folly,constantine001/folly,lifei/folly,arg0/folly,Orvid/folly,gaoyingie/folly,Eagle-X/folly,gavioto/folly,brunomorishita/folly,tomhughes/folly,Hincoin/folly,stonegithubs/folly,gaoyingie/folly,cole14/folly,gavioto/folly,romange/folly,romange/folly,sakishum/folly,Eagle-X/folly,unmeshvrije/C--,bikong2/folly,loversInJapan/folly,juniway/folly,project-zerus/folly,bobegir/folly,wildinto/folly,bikong2/folly,loverszhaokai/folly,ykarlsbrun/folly,zhiweicai/folly,loverszhaokai/folly,ykarlsbrun/folly,kernelim/folly,tempbottle/folly,kernelim/folly,cppfool/folly,bowlofstew/folly,PPC64/folly,reddit/folly,zhiweicai/folly,clearlylin/folly,renyinew/folly,gaoyingie/folly,colemancda/folly,doctaweeks/folly,ajtulloch/folly,clearlylin/folly,loversInJapan/folly,theiver9827/folly,sonnyhu/folly,clearlinux/folly,PoisonBOx/folly,wildinto/folly,stonegithubs/folly,juniway/folly,facebook/folly,juniway/folly,tomhughes/folly,gavioto/folly,unmeshvrije/C--,cppfool/folly,sonnyhu/folly,stonegithubs/folly,bikong2/folly,bobegir/folly,nickhen/folly,SeanRBurton/folly,unmeshvrije/C--,tempbottle/folly,CJstar/folly,ajtulloch/folly,xzmagic/folly,project-zerus/folly,doctaweeks/folly,chjp2046/folly,SeanRBurton/folly,chjp2046/folly,chjp2046/folly,leolujuyi/folly,renyinew/folly,constantine001/folly,facebook/folly,charsyam/folly,leolujuyi/folly,doctaweeks/folly,reddit/folly,CJstar/folly,arg0/folly,raphaelamorim/folly,PPC64/folly,gavioto/folly,romange/folly,rklabs/folly,romange/folly,Hincoin/folly,sonnyhu/folly,wildinto/folly,PoisonBOx/folly,constantine001/folly,ykarlsbrun/folly,mqeizi/folly,yangjin-unique/folly,doctaweeks/folly,sakishum/folly,shaobz/folly,SammyK/folly,guker/folly,fw1121/folly,stonegithubs/folly,clearlinux/folly