refactor(init/meta/lean/parser): simplify sep_by
refactor(init/meta/lean/parser): simplify sep_by

avigad/lean,leodemoura/lean,johoelzl/lean,rlewis1988/lean,leanprover-community/lean,dselsam/lean,digama0/lean,dselsam/lean,avigad/lean,rlewis1988/lean,leodemoura/lean,leodemoura/lean,leanprover/lean,digama0/lean,fpvandoorn/lean,avigad/lean,Kha/lean,leodemoura/lean,Kha/lean,fpvandoorn/lean,dselsam/lean,digama0/lean,dselsam/lean,dselsam/lean,leanprover/lean,Kha/lean,avigad/lean,johoelzl/lean,johoelzl/lean,dselsam/lean,leanprover-community/lean,leanprover/lean,leanprover-community/lean,rlewis1988/lean,rlewis1988/lean,johoelzl/lean,leanprover-community/lean,Kha/lean,Kha/lean,Kha/lean,leanprover/lean,digama0/lean,digama0/lean,leanprover-community/lean,johoelzl/lean,digama0/lean,leanprover/lean,rlewis1988/lean,leanprover/lean,avigad/lean,fpvandoorn/lean,leodemoura/lean,leanprover-community/lean,rlewis1988/lean,avigad/lean,fpvandoorn/lean,johoelzl/lean,fpvandoorn/lean,leodemoura/lean,fpvandoorn/lean