added impelementation-candidate for magic type-erasure nested class

Function.hpp

@@ -11,5 +11,16 @@ private:
     public:
         virtual Ret call(Args...) = 0;
     };
+
+    template <typename Fun>
+    class callable : public callable_base
+    {
+    private:
+        Fun _f;
+    public:
+        callable(Fun f):_f(f) {}
+        virtual Ret call(Args... args)
+        { return _f(std::forward<Args>(args)...); }
+    };
 public:
 };