Add #many? implementations for Scope and AssociationCollection

If anybody has a few spare cycles, I could use a few more reviewers
for this patch:

Basically, when we added Enumerable#many? for 2.2, we didn't add
corresponding methods to Scope and AssociationCollection so that
they'd use a SQL COUNT instead of loading the entire collection.
#any?, #none?, and friends work that way, which is much more efficient
for some use cases.

Chris Kampmeier