Memoize declared fields to avoid reflection during case class decomposition

This change uses the memoize pattern in Meta to cache class fields avoiding reflection on subsequent calls.  

The change discussion on the mailing list:
https://groups.google.com/forum/#!topic/liftweb/O9DDgSZqkkc

Contributor file signature:
https://github.com/lift/framework/blob/master/contributors.md#name-23