add Class#inherited hook

src/class.c

@@ -217,8 +217,12 @@ mrb_vm_define_class(mrb_state *mrb, mrb_value outer, mrb_value super, mrb_sym id
       mrb_check_type(mrb, super, MRB_TT_CLASS);
       s = mrb_class_ptr(super);
     }
+    if (!s) {
+      s = mrb->object_class;
+    }
     c = mrb_class_new(mrb, s);
     setup_class(mrb, outer, c, id);
+    mrb_funcall(mrb, mrb_obj_value(s), "inherited", 1, mrb_obj_value(c));
   }
 
   return c;
@@ -1075,6 +1079,7 @@ mrb_init_class(mrb_state *mrb)
   mrb_define_method(mrb, bob, "!", mrb_bob_not, ARGS_NONE());
   mrb_define_method(mrb, bob, "method_missing", mrb_bob_missing, ARGS_ANY());     /* 15.3.1.3.30 */
   mrb_define_method(mrb, cls, "new", mrb_instance_new, ARGS_ANY());
+  mrb_define_method(mrb, cls, "inherited", mrb_bob_init, ARGS_REQ(1));
   mrb_define_method(mrb, mod, "include", mrb_mod_include, ARGS_REQ(1));
 
   mrb_define_method(mrb, mod, "to_s", mrb_mod_to_s, ARGS_NONE());