Add support for gecko_constant_prefix kwarg for Keyword.
Add support for gecko_constant_prefix kwarg for Keyword.

This allows us to auto-generate settings when the gecko naming is slightly
different than the obvious auto-generated thing.

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