Add a very simple test program

Not quite a unit test, it looks more like an example
but still something.

include/wrenpp/vm.hpp

@@ -104,6 +104,8 @@ namespace wren {
 		void set_user_data (std::nullptr_t);
 		bool has_user_data() const;
 
+		std::size_t dynafunc_byte_size() const;
+
 	private:
 		struct LocalData;