introspection/name: add member_offset query for pointer-to-member

introspection/name.hpp

@@ -168,6 +168,15 @@ namespace cruft::introspection::name {
     }
 
 
+    constexpr std::size_t
+    member_offset (auto ptr)
+    {
+        using klass_t = typename pointer_to_member_class<decltype (ptr)>::type;
+        klass_t instance {};
+        return std::uintptr_t (&(instance.*ptr)) - std::uintptr_t (&instance);
+    }
+
+
     ///////////////////////////////////////////////////////////////////////////
     template <typename T>
     struct type_char;