暂存旧符号表结构定义，TODO.md中添加相关说明

olddef.h

@@ -0,0 +1,62 @@
+
+class SymbolTable{
+private:
+  enum Kind
+  {
+    kModule,
+    kFunction,
+    kBlock,
+  };
+
+  std::forward_list<std::pair<Kind, std::unordered_map<std::string, Value*>>> Scopes;
+
+public:
+  struct ModuleScope {
+    SymbolTable& tables_ref;
+    ModuleScope(SymbolTable& tables) : tables_ref(tables) {
+      tables.enter(kModule);
+    }
+    ~ModuleScope() { tables_ref.exit(); }
+  };
+  struct FunctionScope {
+    SymbolTable& tables_ref;
+    FunctionScope(SymbolTable& tables) : tables_ref(tables) {
+      tables.enter(kFunction);
+    }
+    ~FunctionScope() { tables_ref.exit(); }
+  };
+  struct BlockScope {
+    SymbolTable& tables_ref;
+    BlockScope(SymbolTable& tables) : tables_ref(tables) {
+      tables.enter(kBlock);
+    }
+    ~BlockScope() { tables_ref.exit(); }
+  };
+
+  SymbolTable() = default;
+
+  bool isModuleScope() const { return Scopes.front().first == kModule; }
+  bool isFunctionScope() const { return Scopes.front().first == kFunction; }
+  bool isBlockScope() const { return Scopes.front().first == kBlock; }
+  Value *lookup(const std::string &name) const {
+    for (auto &scope : Scopes) {
+      auto iter = scope.second.find(name);
+      if (iter != scope.second.end())
+        return iter->second;
+    }
+    return nullptr;
+  }
+  auto insert(const std::string &name, Value *value) {
+    assert(not Scopes.empty());
+    return Scopes.front().second.emplace(name, value);
+  }
+private:
+  void enter(Kind kind) {
+    Scopes.emplace_front();
+    Scopes.front().first = kind;
+  }
+  void exit() {
+    Scopes.pop_front(); 
+  }
+  
+};