swift234

Commit next revision of Set<T> public API

The public API is mostly solid at this point, with only a few minor
changes to naming and a couple of extra methods to bring it to parity
with Array.

Also removed the quick-and-dirty operators <, >, <=, and >= which were
used as a convenient shorthand for strict subset, strict superset,
subset, and superset respectively (< is retained for Comparable
conformance).

Swift SVN r23657