should seal the concurrency bug: the map was being set to None, resulting
in potentially two threads servicing the same queue and making it impossible
to reliably create a ClientStorage once the main loop is running.

items expensive.

nodes.

Optimized pickle formats (using tuple rather than tuple of tuples).

Updates Interfaces file with set operations and byValue method.

(re)Implemented new set operations and added byValue method.

Fixed bug in handling of empty BTreeItems.

Rewrote suite building to use a test_suite function as required by
Brian's test runner.

Added a debug function and a new __main__ handler that can run it.

Added flags to prevent subclasses from having instance dictionaries.
It's not clear that people will actually subclass these things and
supporting instance dictionaries adds extra effort at this time, which
we can't afford.

comparison yet.

Fixed typo in length test.

Changed wording of error when a nonundoable transaction is attempted to be undone from "undoable transaction" to "nonundoable transaction"

Changed wording of UndoError docstring from "an attempt was made to undo an undoable transaction" to "an attempt was made to undo a non-undoable transaction"