Implement native extension system

* Deprecates GDNative in favor of a simpler, lower level interface.
* New extension system allows registering core engine classes.
* Simple header interface in gdnative_interace.h

core/object/method_bind.h

@@ -135,6 +135,8 @@ public:
 
 	void set_default_arguments(const Vector<Variant> &p_defargs);
 
+	uint32_t get_hash() const;
+
 	MethodBind();
 	virtual ~MethodBind();
 };