Switch to using ContravariantLifetime in JSRef<'a, T>.
Switch to using ContravariantLifetime in JSRef<'a, T>.

Since ContravariantLifetime doesn't take up any storage space, this
means that JSRef will be a single word. This fixes #2333.

bfrohs/servo,aweinstock314/servo,seanmonstar/servo,huonw/servo,ruud-v-a/servo,notriddle/servo,pyfisch/servo,peterjoel/servo,GreenRecycleBin/servo,brendandahl/servo,hyowon/servo,szeged/servo,srbhklkrn/SERVOENGINE,A-deLuna/servo,caldwell/servo,brendandahl/servo,aweinstock314/servo,SimonSapin/servo,akosel/servo,mattnenterprise/servo,DominoTree/servo,ryancanhelpyou/servo,AnthonyBroadCrawford/servo,sadmansk/servo,ConnorGBrewster/servo,vks/servo,Shraddha512/servo,canaltinova/servo,aweinstock314/servo,RenaudParis/servo,mattnenterprise/servo,brendandahl/servo,dmarcos/servo,boghison/servo,juzer10/servo,saneyuki/servo,snf/servo,shrenikgala/servo,eddyb/servo,canaltinova/servo,caldwell/servo,szeged/servo,aweinstock314/servo,ruud-v-a/servo,nick-thompson/servo,emilio/servo,Adenilson/prototype-viewing-distance,saratang/servo,dati91/servo,aidanhs/servo,echochamber/servo,paulrouget/servo,ryancanhelpyou/servo,jimberlage/servo,SimonSapin/servo,nrc/servo,KiChjang/servo,meh/servo,GyrosOfWar/servo,snf/servo,j3parker/servo,youprofit/servo,Adenilson/prototype-viewing-distance,CJ8664/servo,walac/servo,rnestler/servo,steveklabnik/servo,walac/servo,echochamber/servo,s142857/servo,GyrosOfWar/servo,g-k/servo,rixrix/servo,WriterOfAlicrow/servo,indykish/servo,akosel/servo,j3parker/servo,sadmansk/servo,saneyuki/servo,wpgallih/servo,nnethercote/servo,pyecs/servo,WriterOfAlicrow/servo,tafia/servo,evilpie/servo,wartman4404/servo,peterjoel/servo,j3parker/servo,A-deLuna/servo,samfoo/servo,WriterOfAlicrow/servo,cbrewster/servo,brendandahl/servo,fiji-flo/servo,juzer10/servo,wpgallih/servo,bjwbell/servo,nick-thompson/servo,walac/servo,splav/servo,akosel/servo,aidanhs/servo,GreenRecycleBin/servo,A-deLuna/servo,Shraddha512/servo,SimonSapin/servo,canaltinova/servo,zentner-kyle/servo,KiChjang/servo,peterjoel/servo,s142857/servo,dmarcos/servo,nrc/servo,mukilan/servo,KiChjang/servo,pyfisch/servo,sadmansk/servo,zentner-kyle/servo,mdibaiee/servo,pyecs/servo,CJ8664/servo,samfoo/servo,larsbergstrom/servo,GyrosOfWar/servo,avadacatavra/servo,michaelwu/servo,CJ8664/servo,GyrosOfWar/servo,huonw/servo,karlito40/servo,michaelwu/servo,avadacatavra/servo,dmarcos/servo,GreenRecycleBin/servo,paulrouget/servo,jlegendary/servo,juzer10/servo,youprofit/servo,hyowon/servo,nerith/servo,jdramani/servo,mt2d2/servo,indykish/servo,evilpie/servo,cbrewster/servo,GyrosOfWar/servo,samfoo/servo,ruud-v-a/servo,rixrix/servo,mattnenterprise/servo,avadacatavra/servo,srbhklkrn/SERVOENGINE,chotchki/servo,pyfisch/servo,notriddle/servo,brendandahl/servo,shrenikgala/servo,pyfisch/servo,dhananjay92/servo,froydnj/servo,rixrix/servo,deokjinkim/servo,aidanhs/servo,eddyb/servo,michaelwu/servo,deokjinkim/servo,akosel/servo,codemac/servo,mt2d2/servo,jimberlage/servo,mattnenterprise/servo,dhananjay92/servo,thiagopnts/servo,jimberlage/servo,jlegendary/servo,zentner-kyle/servo,rentongzhang/servo,dagnir/servo,nerith/servo,j3parker/servo,bjwbell/servo,mukilan/servo,luniv/servo,avadacatavra/servo,wartman4404/servo,mdibaiee/servo,CJ8664/servo,caldwell/servo,zhangjunlei26/servo,paulrouget/servo,ryancanhelpyou/servo,anthgur/servo,kindersung/servo,deokjinkim/servo,splav/servo,cbrewster/servo,RenaudParis/servo,ryancanhelpyou/servo,fiji-flo/servo,karlito40/servo,jimberlage/servo,shrenikgala/servo,ruud-v-a/servo,saneyuki/servo,boghison/servo,dsandeephegde/servo,indykish/servo,AnthonyBroadCrawford/servo,walac/servo,pgonda/servo,boghison/servo,meh/servo,kindersung/servo,aidanhs/servo,jlegendary/servo,larsbergstrom/servo,michaelwu/servo,saneyuki/servo,echochamber/servo,dhananjay92/servo,zhangjunlei26/servo,mbrubeck/servo,szeged/servo,nerith/servo,akosel/servo,zentner-kyle/servo,dati91/servo,Shraddha512/servo,mt2d2/servo,indykish/servo,A-deLuna/servo,g-k/servo,nnethercote/servo,zhangjunlei26/servo,juzer10/servo,shrenikgala/servo,anthgur/servo,dvberkel/servo,youprofit/servo,splav/servo,codemac/servo,walac/servo,rixrix/servo,jimberlage/servo,nrc/servo,michaelwu/servo,ConnorGBrewster/servo,nnethercote/servo,sadmansk/servo,eddyb/servo,seanmonstar/servo,DominoTree/servo,j3parker/servo,dvberkel/servo,jimberlage/servo,nick-thompson/servo,fiji-flo/servo,indykish/servo,g-k/servo,szeged/servo,ConnorGBrewster/servo,seanmonstar/servo,dvberkel/servo,kindersung/servo,thiagopnts/servo,deokjinkim/servo,rixrix/servo,codemac/servo,szeged/servo,j3parker/servo,dvberkel/servo,peterjoel/servo,srbhklkrn/SERVOENGINE,szeged/servo,DominoTree/servo,szeged/servo,echochamber/servo,jdramani/servo,froydnj/servo,mbrubeck/servo,mattnenterprise/servo,larsbergstrom/servo,deokjinkim/servo,zentner-kyle/servo,ConnorGBrewster/servo,indykish/servo,saratang/servo,snf/servo,AnthonyBroadCrawford/servo,samfoo/servo,snf/servo,paulrouget/servo,bfrohs/servo,pyecs/servo,akosel/servo,hyowon/servo,tafia/servo,luniv/servo,dati91/servo,KiChjang/servo,rentongzhang/servo,evilpie/servo,srbhklkrn/SERVOENGINE,aidanhs/servo,splav/servo,huonw/servo,dagnir/servo,wpgallih/servo,dagnir/servo,mukilan/servo,ryancanhelpyou/servo,cbrewster/servo,pyfisch/servo,saneyuki/servo,pet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/servo,Shraddha512/servo,runarberg/servo,mbrubeck/servo,wpgallih/servo,rnestler/servo,huonw/servo,Adenilson/prototype-viewing-distance,splav/servo,froydnj/servo,rnestler/servo,ConnorGBrewster/servo,mattnenterprise/servo,larsbergstrom/servo,kindersung/servo,snf/servo,s142857/servo,Shraddha512/servo,tempbottle/servo,thiagopnts/servo,wartman4404/servo,WriterOfAlicrow/servo,sadmansk/servo,mukilan/servo,shrenikgala/servo,paulrouget/servo,splav/servo,mattnenterprise/servo,larsbergstrom/servo,rnestler/servo,rnestler/servo,eddyb/servo,Adenilson/prototype-viewing-distance,cbrewster/servo,sadmansk/servo,Shraddha512/servo,froydnj/servo,karlito40/servo,tschneidereit/servo,rentongzhang/servo,runarberg/servo,Shraddha512/servo,jlegendary/servo,kindersung/servo,samfoo/servo,bfrohs/servo,youprofit/servo,ryancanhelpyou/servo,dhananjay92/servo,nrc/servo,rixrix/servo,luniv/servo,DominoTree/servo,sadmansk/servo,huonw/servo,eddyb/servo,SimonSapin/servo,nrc/servo,mdibaiee/servo,codemac/servo,rnestler/servo,jgraham/servo,SimonSapin/servo,bfrohs/servo,SimonSapin/servo,chotchki/servo,huonw/servo,jgraham/servo,zhangjunlei26/servo,wpgallih/servo,g-k/servo,anthgur/servo,GreenRecycleBin/servo,mukilan/servo,meh/servo,wpgallih/servo,paulrouget/servo,dvberkel/servo,vks/servo,anthgur/servo,steveklabnik/servo,anthgur/servo,saratang/servo,brendandahl/servo,caldwell/servo,pyecs/servo,KiChjang/servo,DominoTree/servo,wpgallih/servo,runarberg/servo,nerith/servo,bjwbell/servo,splav/servo,rentongzhang/servo,youprofit/servo,larsbergstrom/servo,michaelwu/servo,larsbergstrom/servo,eddyb/servo,CJ8664/servo,pgonda/servo,bfrohs/servo,pyfisch/servo,dsandeephegde/servo,caldwell/servo,youprofit/servo,seanmonstar/servo,aidanhs/servo,mt2d2/servo,dsandeephegde/servo,SimonSapin/servo,dvberkel/servo,notriddle/servo,kindersung/servo,RenaudParis/servo,mbrubeck/servo,hyowon/servo,ConnorGBrewster/servo,szeged/servo,notriddle/servo,codemac/servo,KiChjang/servo,wartman4404/servo,meh/servo,mdibaiee/servo,jgraham/servo,canaltinova/servo,tempbottle/servo,tschneidereit/servo,ConnorGBrewster/servo,saratang/servo,aweinstock314/servo,saneyuki/servo,wartman4404/servo,szeged/servo,KiChjang/servo,pgonda/servo,avadacatavra/servo,saneyuki/servo