added 'replace' function to Slab that works like std::mem::replace
added 'replace' function to Slab that works like std::mem::replace

fuchsia-mirror/third_party-mio,frewsxcv/mio,tempbottle/mio,asomers/mio,rrichardson/mio,arthurprs/mio,mneumann/mio,alexcrichton/mio,mneumann/mio,mlalic/mio,yangxingpping/mio,hjr3/mio,luca-barbieri/mio,murarth/mio,dpc/mio,danburkert/mio,fhartwig/mio,pythonesque/mio,carllerche/mio,echochamber/mio,greensnark/mio,yuikns/mio,marjakm/mio,asomers/mio,carllerche/mio,zslayton/mio,rryan/mio,fuchsia-mirror/third_party-mio,thijsc/mio,bottompawn/mio,tailhook/mio,geofft/mio,dwrensha/mio