added interface for a nested class which could save us all

Function.hpp

@@ -5,5 +5,11 @@ class Function;
 template <typename Ret, typename... Args>
 class function
 {
-    // TODO: everything :'D
+private:
+    class callable_base
+    {
+    public:
+        virtual Ret call(Args...) = 0;
+    };
+public:
 };