fix(library/vm/vm_string): bug at string.fold VM builtin implementation
fix(library/vm/vm_string): bug at string.fold VM builtin implementation

leodemoura/lean,dselsam/lean,fpvandoorn/lean,Kha/lean,dselsam/lean,digama0/lean,Kha/lean,dselsam/lean,dselsam/lean,leanprover-community/lean,fpvandoorn/lean,leanprover/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover/lean,digama0/lean,leanprover-community/lean,dselsam/lean,digama0/lean,Kha/lean,leodemoura/lean,dselsam/lean,leanprover-community/lean,leodemoura/lean,leanprover/lean,fpvandoorn/lean,leanprover/lean,Kha/lean,digama0/lean,leanprover-community/lean,leanprover-community/lean,leanprover/lean,Kha/lean,leanprover/lean,leodemoura/lean,digama0/lean,leanprover-community/lean,fpvandoorn/lean,leodemoura/lean,Kha/lean,leodemoura/lean,digama0/lean