backend

I've posted a first cut at a patch to refactor core search into a generic search module that accepts different backend handlers and a specific handler, core's custom indexing. It's a very basic patch--it just does the minimum to decouple the generic search interface from the backend.

The aim is to permit e.g. apachesolr and other search backends to plug in seamlessly to core search.

Ready for review: http://drupal.org/node/282192.