* Improve code for the empty frozenset singleton:

* Improve code for the empty frozenset singleton:
  - Handle both frozenset() and frozenset([]).
  - Do not use singleton for frozenset subclasses.
  - Finalize the singleton.
  - Add test cases.
* Factor-out set_update_internal() from set_update().  Simplifies the
  code for several internal callers.
* Factor constant expressions out of loop in set_merge_internal().
* Minor comment touch-ups.