message: das.Symbol: Add `method_signature`.
diff: return S::template _method_has<T, Args...>(0); \
  } \
  \
  + template <typename T> \
  + using method_signature = \
  + typename std::remove_class<decltype(&T::CName)>::type; \
  + \
  template <typename T, typename ... Args> \
  using method_type \
  = decltype(std::declval<T&>() \
  