- Patch #35768 by JonBob/Richard: separate callback handling from menu handling.

  Moves callbacks and arguments to a separate data structure from the visible menu tree. As mentioned on drupal-devel, this change has little impact except to slightly improve code legibility and prepare the code base for future separation of these tasks, with an eye toward performance improvements of the menu system.  Also changes array_key_exists() to isset() in several places, which is reported to perform better.