pointer, with undefined results and a compiler warning.

- Fixed: the Python implementation of the _p_estimated_size propety
  didn't support deletion.

- Simplified implememnntation of the _p_estimated_size property to
  only accept integers.  A TypeError is raised if an incorrect type is
  provided.

Avoids clash w/ pure-Python 'timestamp' module on case-insensitive filesystems.

N.B:  the canonical way to import the ``TimeStamp`` class is now::

  from persistent.timestamp import TimeStamp

which will yield the class from the extension module (if available),
falling back to the pure-Python reference implementation.

Fix a couple of discrepancies in the Python persistence implementation
uncovered thereby.

The doctest snippets moved to docs/api/attributes.