Fix https://github.com/julianhyde/optiq/issues/114, "Short-cut query if LIMIT 0 is present".

Add rules to short-cut a query if LIMIT 0 is present. Also remove sort, aggregation, join if their inputs are known to be empty, and propagate the fact that the relational expressions are known to be empty up the tree. (We already do this for union, filter, project.)

Rename RemoveEmptyRule to RemoveEmptyRules, since it is no longer a rule, just a namespace for other rules.
Obsolete our MultiMap; use Guava's Multimap instead.