Use the unstable feature of string_cache and selectors in geckolib
Use the unstable feature of string_cache and selectors in geckolib

This should enable Cargo to reuse more build artifacts between servo and geckolib.

dati91/servo,anthgur/servo,DominoTree/servo,szeged/servo,eddyb/servo,saneyuki/servo,ConnorGBrewster/servo,anthgur/servo,CJ8664/servo,DominoTree/servo,dati91/servo,pyfisch/servo,steveklabnik/servo,eddyb/servo,rnestler/servo,steveklabnik/servo,pyfisch/servo,sadmansk/servo,rnestler/servo,mattnenterprise/servo,KiChjang/servo,notriddle/servo,mattnenterprise/servo,emilio/servo,saneyuki/servo,paulrouget/servo,notriddle/servo,jimberlage/servo,jimberlage/servo,nnethercote/servo,KiChjang/servo,steveklabnik/servo,fiji-flo/servo,jimberlage/servo,KiChjang/servo,pyfisch/servo,mbrubeck/servo,KiChjang/servo,mattnenterprise/servo,fiji-flo/servo,sadmansk/servo,notriddle/servo,nnethercote/servo,mbrubeck/servo,notriddle/servo,splav/servo,splav/servo,szeged/servo,nnethercote/servo,jimberlage/servo,emilio/servo,cbrewster/servo,rnestler/servo,eddyb/servo,dati91/servo,fiji-flo/servo,KiChjang/servo,nnethercote/servo,anthgur/servo,szeged/servo,saneyuki/servo,SimonSapin/servo,notriddle/servo,avadacatavra/servo,SimonSapin/servo,CJ8664/servo,saneyuki/servo,dsandeephegde/servo,paulrouget/servo,nrc/servo,DominoTree/servo,fiji-flo/servo,CJ8664/servo,cbrewster/servo,peterjoel/servo,canaltinova/servo,SimonSapin/servo,fiji-flo/servo,mbrubeck/servo,saneyuki/servo,szeged/servo,dsandeephegde/servo,thiagopnts/servo,mattnenterprise/servo,steveklabnik/servo,nnethercote/servo,nrc/servo,dati91/servo,nnethercote/servo,saneyuki/servo,avadacatavra/servo,splav/servo,KiChjang/servo,canaltinova/servo,avadacatavra/servo,emilio/servo,dsandeephegde/servo,canaltinova/servo,CJ8664/servo,cbrewster/servo,mbrubeck/servo,SimonSapin/servo,paulrouget/servo,avadacatavra/servo,larsbergstrom/servo,eddyb/servo,pyfisch/servo,dsandeephegde/servo,sadmansk/servo,splav/servo,dati91/servo,ConnorGBrewster/servo,saneyuki/servo,upsuper/servo,larsbergstrom/servo,nnethercote/servo,splav/servo,peterjoel/servo,rnestler/servo,cbrewster/servo,cbrewster/servo,KiChjang/servo,steveklabnik/servo,jimberlage/servo,larsbergstrom/servo,ConnorGBrewster/servo,ConnorGBrewster/servo,sadmansk/servo,szeged/servo,notriddle/servo,mbrubeck/servo,anthgur/servo,DominoTree/servo,CJ8664/servo,KiChjang/servo,dsandeephegde/servo,canaltinova/servo,CJ8664/servo,thiagopnts/servo,nrc/servo,DominoTree/servo,SimonSapin/servo,paulrouget/servo,jimberlage/servo,dsandeephegde/servo,emilio/servo,upsuper/servo,thiagopnts/servo,peterjoel/servo,rnestler/servo,paulrouget/servo,anthgur/servo,canaltinova/servo,nrc/servo,szeged/servo,emilio/servo,dati91/servo,ConnorGBrewster/servo,ConnorGBrewster/servo,peterjoel/servo,peterjoel/servo,splav/servo,ConnorGBrewster/servo,mbrubeck/servo,thiagopnts/servo,emilio/servo,notriddle/servo,upsuper/servo,nnethercote/servo,larsbergstrom/servo,splav/servo,nrc/servo,cbrewster/servo,steveklabnik/servo,rnestler/servo,peterjoel/servo,thiagopnts/servo,larsbergstrom/servo,upsuper/servo,peterjoel/servo,DominoTree/servo,avadacatavra/servo,SimonSapin/servo,eddyb/servo,szeged/servo,upsuper/servo,paulrouget/servo,upsuper/servo,fiji-flo/servo,canaltinova/servo,mattnenterprise/servo,thiagopnts/servo,notriddle/servo,KiChjang/servo,saneyuki/servo,emilio/servo,emilio/servo,dati91/servo,saneyuki/servo,anthgur/servo,upsuper/servo,canaltinova/servo,jimberlage/servo,emilio/servo,szeged/servo,szeged/servo,pyfisch/servo,paulrouget/servo,sadmansk/servo,DominoTree/servo,anthgur/servo,canaltinova/servo,dsandeephegde/servo,thiagopnts/servo,SimonSapin/servo,sadmansk/servo,mattnenterprise/servo,pyfisch/servo,jimberlage/servo,nrc/servo,fiji-flo/servo,avadacatavra/servo,mbrubeck/servo,szeged/servo,saneyuki/servo,jimberlage/servo,nnethercote/servo,cbrewster/servo,pyfisch/servo,DominoTree/servo,peterjoel/servo,splav/servo,mattnenterprise/servo,paulrouget/servo,larsbergstrom/servo,larsbergstrom/servo,emilio/servo,notriddle/servo,paulrouget/servo,larsbergstrom/servo,anthgur/servo,dsandeephegde/servo,notriddle/servo,DominoTree/servo,rnestler/servo,pyfisch/servo,sadmansk/servo,splav/servo,paulrouget/servo,steveklabnik/servo,mbrubeck/servo,CJ8664/servo,rnestler/servo,avadacatavra/servo,dati91/servo,ConnorGBrewster/servo,CJ8664/servo,eddyb/servo,sadmansk/servo,peterjoel/servo,avadacatavra/servo,eddyb/servo,larsbergstrom/servo,peterjoel/servo,pyfisch/servo,cbrewster/servo,eddyb/servo,larsbergstrom/servo,DominoTree/servo,splav/servo,pyfisch/servo,nnethercote/servo,upsuper/servo,KiChjang/servo,thiagopnts/servo,mattnenterprise/servo,nrc/servo,fiji-flo/servo,SimonSapin/servo