refactor(frontends/lean/parser): add missing 'const'
refactor(frontends/lean/parser): add missing 'const'

leanprover-community/lean,rlewis1988/lean,sp3ctum/lean,avigad/lean,soonhokong/lean-osx,digama0/lean,rlewis1988/lean,htzh/lean,fgdorais/lean,fpvandoorn/lean,UlrikBuchholtz/lean,sp3ctum/lean,soonhokong/lean-osx,soonhokong/lean,fpvandoorn/lean,fpvandoorn/lean2,fpvandoorn/lean,Kha/lean,leanprover/lean,fpvandoorn/lean2,leanprover/lean,soonhokong/lean-windows,Kha/lean,rlewis1988/lean,leanprover-community/lean,johoelzl/lean,UlrikBuchholtz/lean,leodemoura/lean,htzh/lean,javra/lean,johoelzl/lean,fgdorais/lean,dselsam/lean,javra/lean,fpvandoorn/lean,leanprover-community/lean,javra/lean,johoelzl/lean,c-cube/lean,digama0/lean,leodemoura/lean,leanprover/lean,dselsam/lean,soonhokong/lean-windows,Kha/lean,UlrikBuchholtz/lean,Kha/lean,fpvandoorn/lean,leanprover/lean,leanprover/lean,avigad/lean,levnach/lean,UlrikBuchholtz/lean,eigengrau/lean,fpvandoorn/lean2,digama0/lean,leodemoura/lean,fgdorais/lean,leodemoura/lean,leodemoura/lean,levnach/lean,avigad/lean,dselsam/lean,sp3ctum/lean,htzh/lean,eigengrau/lean,eigengrau/lean,soonhokong/lean-windows,dselsam/lean,avigad/lean,soonhokong/lean-osx,leanprover-community/lean,rlewis1988/lean,UlrikBuchholtz/lean,soonhokong/lean,leanprover/lean,johoelzl/lean,avigad/lean,johoelzl/lean,levnach/lean,soonhokong/lean,htzh/lean,rlewis1988/lean,soonhokong/lean-windows,javra/lean,leanprover-community/lean,dselsam/lean,fgdorais/lean,soonhokong/lean,fpvandoorn/lean2,digama0/lean,rlewis1988/lean,Kha/lean,javra/lean,sp3ctum/lean,c-cube/lean,soonhokong/lean-windows,leanprover-community/lean,eigengrau/lean,fgdorais/lean,digama0/lean,avigad/lean,Kha/lean,c-cube/lean,sp3ctum/lean,soonhokong/lean-osx,fpvandoorn/lean,levnach/lean,htzh/lean,soonhokong/lean-osx,fgdorais/lean,dselsam/lean,c-cube/lean,digama0/lean,levnach/lean,eigengrau/lean,fpvandoorn/lean2,soonhokong/lean,c-cube/lean,johoelzl/lean,leodemoura/lean