Menus are now configuration entities

As a part of #1802750: [Meta] Convert configurable data to ConfigEntity system

The {menu_custom} table has been removed in favor of a new menu config entity. The table is preserved in installations upgraded from earlier versions of Drupal to allow contrib modules to migrate their data, See #1860986: Drop left-over tables from 8.x

Summary

• $menu becomes a classed object, Drupal\system\Plugin\Core\Entity\Menu.
• Wherever the code refers to the menu_name, use $menu->id().
• Wherever the code intends to output a name for the menu, use $menu->label().

API changes

• field_ui_menu_load() renamed to field_ui_instance_load() to not fire hook_menu_load() as implementation of hook_ENTITY_TYPE_load(). Contrib modules should check their function names too.
• Removed menu_load_all(), menu_save() and menu_delete() use entity API methods instead.
• All hook_menu_*() hooks now receive \Drupal\system\Plugin\Core\Entity\Menu $menu as parameter and just a hook_ENTITY_TYPE_*() hooks
• Menu add/edit forms now use the entity form controller MenuFormController.
• Administration page uses the entity list controller MenuListController, menu_admin_overview() has been removed.
• As do all configuration entities, menus now have a langcode property.

Examples

// D7 code
$menus = menu_load_all();
foreach ($menus as $menu) {
//...

// D8 code
$menus = Menu::loadMultiple();
foreach ($menus as $menu) {
//...

Entity API

// D7 code
$menu = array(
  'menu_name' => $menu_machine_name,
  'title' => 'Custom menu human-readable name',
  'description' => 'Description text',
);
menu_save($menu);

// D8 code
$menu = Menu::create(array(
  'id' => $menu_machine_name,
  'label' => 'Custom menu human-readable name',
  'description' => 'Description text',
));
$menu->save();

Hooks

// D7 code
function hook_menu_delete($menu) {
  // Delete the record from our variable.
  $my_menus = variable_get('my_module_menus', array());
  unset($my_menus[$menu['menu_name']]);
  variable_set('my_module_menus', $my_menus);
}

// D8 code
function hook_menu_delete($menu) {
  // Delete the record from our variable.
  $my_menus = variable_get('my_module_menus', array());
  unset($my_menus[$menu->id()]);
  variable_set('my_module_menus', $my_menus);
}