Fix destroy() on hash objects. It currently does not remove the object,
but empties it incorrectly.

Add `limit' option to searches to limit the number of results retrieved.

Negation of 'wq' and 'ws' in search() (Marcos' idea)

fill() method.

temporary_data() method to get per-object space to store something
temporary that would not be stored in the object on disk. Some cached
values or something like that. Just a hash or SimpleHash reference in
fact.
