Include nav menu caching

[emrikol]

With a large menu structure (and maybe a custom walker or two) nav menus can take quite a long time to generate, and they will be regenerated on every page.

VIP has a menu caching plugin for WordPress.com that can probably be easily made compatible with WordPress.org installs

[sboisvert]

I think we should add this and also add parts of the sidebar / footer / header caching plugin that I saw once and can't find again.... odds are we can recreate most of it pretty quickly. #famouslastwords

[emrikol]

Would it be better to just cache wp_nav_menu and not use a helper function?

I think we can use the pre_wp_nav_menu and wp_nav_menu filters to provide caching to the core function.