feat(library/init/to_string): add fin.has_to_string
feat(library/init/to_string): add fin.has_to_string

leanprover-community/lean,avigad/lean,johoelzl/lean,johoelzl/lean,avigad/lean,avigad/lean,fgdorais/lean,avigad/lean,leodemoura/lean,avigad/lean,fgdorais/lean,johoelzl/lean,leanprover/lean,leanprover-community/lean,rlewis1988/lean,avigad/lean,soonhokong/lean-windows,digama0/lean,rlewis1988/lean,leanprover/lean,rlewis1988/lean,Kha/lean,leanprover/lean,dselsam/lean,digama0/lean,rlewis1988/lean,fgdorais/lean,soonhokong/lean-windows,leanprover-community/lean,leanprover-community/lean,johoelzl/lean,digama0/lean,digama0/lean,soonhokong/lean-windows,fpvandoorn/lean,leodemoura/lean,fgdorais/lean,Kha/lean,soonhokong/lean-osx,fpvandoorn/lean,Kha/lean,johoelzl/lean,soonhokong/lean-windows,digama0/lean,fpvandoorn/lean,soonhokong/lean-osx,leanprover/lean,fgdorais/lean,Kha/lean,leodemoura/lean,fpvandoorn/lean,rlewis1988/lean,leodemoura/lean,soonhokong/lean-osx,dselsam/lean,dselsam/lean,fgdorais/lean,rlewis1988/lean,leanprover/lean,soonhokong/lean-osx,dselsam/lean,Kha/lean,soonhokong/lean-osx,leodemoura/lean,leodemoura/lean,fpvandoorn/lean,leanprover/lean,digama0/lean,fpvandoorn/lean,soonhokong/lean-windows,johoelzl/lean,leanprover-community/lean,dselsam/lean,leanprover-community/lean,Kha/lean,dselsam/lean