Splitting off core menu functions into a menu api module

Nice work on the module. It has the most comprehensive menu handling routines of any of the modules that I have reviewed over the last few days.

In the readme, you talk of the menu block api that is exposed by module and I am wondering if you would consider making this a tad more generic and splitting the core menu handling functions into their own module. This would be the first step in reducing the huge amount of duplicate code that has sprung up by modules extending the menu system.