Improve return types.

src/Libs/Middleware.php

@@ -20,7 +20,7 @@ class Middleware {
      *
      * @return mixed
      */
-    public static function next(Neuron $req) {
+    public static function next(Neuron $req): mixed {
         $next = array_pop($req->next);
         return call_user_func_array($next, [$req]);
     }