src: simplify is_callable by making it a concept

Using a C++20 `concept` here makes `is_callable` much simpler
than relying on SFINAE. It is equivalent for function types,
`std::function`, lambdas, and classes with `operator()`,
regardless of argument or return types.

Author: tniessen

src/req_wrap-inl.h

@@ -111,7 +111,7 @@ struct CallLibuvFunction<ReqT, void(*)(ReqT*, Args...)> {
 template <typename ReqT, typename T>
 struct MakeLibuvRequestCallback {
   static T For(ReqWrap<ReqT>* req_wrap, T v) {
-    static_assert(!is_callable<T>::value,
+    static_assert(!is_callable<T>,
                   "MakeLibuvRequestCallback missed a callback");
     return v;
   }

src/util.h

@@ -689,13 +689,9 @@ class NonCopyableMaybe {
 };
 
 // Test whether some value can be called with ().
-template <typename T, typename = void>
-struct is_callable : std::is_function<T> { };
-
 template <typename T>
-struct is_callable<T, typename std::enable_if<
-    std::is_same<decltype(void(&T::operator())), void>::value
-    >::type> : std::true_type { };
+concept is_callable =
+    std::is_function<T>::value || requires(T t) { &T::operator(); };
 
 template <typename T, void (*function)(T*)>
 struct FunctionDeleter {