Rename cooked-index-storage.[ch]

Rename cooked-index-storage.[ch]

A discussion with Simon made me realize that cooked_index_storage
isn't a very clear name, especially now that it's escaped from read.c.
While it does provide some storage (I guess any object does in a
sense), it is really a helper for cooked_index_worker -- a temporary
object that is destroyed after reading has completed.

This patch renames this file.  Later patches will rename the class and
move cooked_index_worker here, something I think is reasonable given
that cooked_index_storage is really something of a helper class for
cooked_index_worker.

Approved-By: Simon Marchi <simon.marchi@efficios.com>