Merge branch '3.4' into 4.2

* 3.4:
  Add missing `@internal` annotations
  Disable Twig in the profiler menu when Twig is not used
  Mark some/most implementations of Serializable as `@internal`
  [Config] ensure moving away from Serializable wont break cache:clear
  [VarDumper] dont implement Serializable in Stub
  [Config] fix compat with wrapping autoloaders

Author: nicolas-grekas

Extension/DataCollector/FormDataCollector.php

@@ -229,6 +229,9 @@ public function getData()
         return $this->data;
     }
 
+    /**
+     * @internal
+     */
     public function serialize()
     {
         foreach ($this->data['forms_by_hash'] as &$form) {

FormError.php

@@ -135,9 +135,7 @@ public function getOrigin()
     }
 
     /**
-     * Serializes this error.
-     *
-     * @return string The serialized error
+     * @internal
      */
     public function serialize()
     {
@@ -151,9 +149,7 @@ public function serialize()
     }
 
     /**
-     * Unserializes a serialized error.
-     *
-     * @param string $serialized The serialized error
+     * @internal
      */
     public function unserialize($serialized)
     {