Created by: wangkuiyi

In class Scope, there are data members:

 private:
  std::unordered_map<Variable*, std::string> var_to_name_;
  std::unordered_map<std::string, std::unique_ptr<Variable>> name_to_var_;

It seems that name_to_var_ is necessary to ensure the uniqueness of variable names in a Scope. But var_to_name_ could be removed if we add Variable::name.

In this way, we don't need Scope::VariableName at all.