function_traits: export the function signature as a type

This makes it easy to further examine using pattern matching instead
of std::index_sequence.

core/function_traits.hh

@@ -31,6 +31,7 @@ struct function_traits<Ret(Args...)>
 {
     using return_type = Ret;
     using args_as_tuple = std::tuple<Args...>;
+    using signature = Ret (Args...);
  
     static constexpr std::size_t arity = sizeof...(Args);