Fixed a few bugs in set/c's keyword options, and one test for set/c.

Specifically, set/c no longer behaves as a flat contract, so it is necessary to
apply at least one set operation to detect a contract error on an element.  It
is possible the flat behavior can be restored for some kinds of sets, but it's
not immediately obvious what kinds those should be.  (Immutable hash sets in
particular?  Any sets that don't implement set-add! or set-remove!?)