Make Menu implement Iterator

[acobster]

I want to be able to do a (depth-first) search through a tree of MenuItems:

$randomPost = new Page(123);
foreach ($menu as $item) {
  if ($item->object_id === $randomPost->id) {
    // do some stuff with $item
  }
}

This is useful for finding items matching arbitrary criteria (that is, other than the "current" menu item, or ancestors of it).

The simplest way to accomplish this is by implementing the Iterator interface in Menu, using an internal stack of items to keep track of position in the depth-first search.

NOTE: an implementation of this lives in the Metro Parks codebase. We just need to backport it and get some tests around it. Tests will likely be a bit of a PITA because Timber's Menu is coupled to the database. There are a few approaches I could see us taking to solve that:

• Support the WP unit testing framwork Support the WordPress "unit" test framework #119
• Use WP_Mock (see SiteTest.php for an example) to mock menu/item data
• Implement syntax in a YAML fixture