New predicates, is_(proper_)?(sub|super)set.

* is_subset
* is_superset
* is_proper_subset
* is_proper_superset

These predicates each operate on two arrayref sets,
and return a value of 0 or 1 (as opposed to '' or 1).

With @logie17, on behalf of @shutterstock.
Merge pull request #1 from twhaples/master

Set-Functional should support predicates like is_subset