The implementation now defers all implementation decisions to UserDict
and merely handles updates to _p_changed in the PersistentMapping
implementation.  This change simplifies the implementation and ensures
that it implements all the new methods of dictionaries like popitem()
and setdefault().

There is one (important?) behavioral change:  The old implementation
of the keys() method did not return keys that were strings starting
with underscore.  This behavior was undocumented and could lead to
problems for implementations that expect keys() to working like a
regular mapping.

    The feature was removed after verifying that the Zope test suite
    does not depend on this behavior.

An internal implementation change is that the PersistentMapping does
not keep a copy of the keys() of the dict.  It simplifies the
implementation a lot to remove the cacheing and the benefit is
unclear.  It only benefits applications that call keys() frequently
without modifying the dict.  And it requires that two copies of the
list exist for each call to keys() which may require a lot of space.