Support serialization of private properties

Problem

Because DependencySerializationTrait uses get_object_vars() to collect serializable service properties, but that function cannot access private properties. Because of that, the code below fails, because $entityTypeManager becomes null when deserialization happens.

<?php

final class MyForm extends \Drupal\Core\Form\FormBase {

  private $entityTypeManager;

  public function __construct(\Drupal\Core\Entity\EntityTypeManagerInterface $entityTypeManager) {
    $this->entityTypeManager = $entityTypeManager;
  }

  public static function create(\Symfony\Component\DependencyInjection\ContainerInterface $container) {
    return new static(
      $container->get('entity_type.manager')
    );
  }
}

This seems to be a known limitation for 2 years or so: https://www.drupal.org/project/drupal/issues/2727011#comment-12619886

Proposed solution

Use Reflection or Clousers instead of get_object_vars().

TODOs

Monitor and measure performance changes: https://www.lambda-out-loud.com/posts/accessing-private-properties-php/