Instead of testing object sizes.

This matches what the pure Python implementation does and is a
stronger test that the object really is compatible with the cache.
Previously, an object could potentially include ``cPersistent_HEAD``
and *not* set ``tp_base`` to ``cPersistenceCAPI->pertype`` and still
be eligible for the pickle cache; that is no longer the case.

This resolves several compiler warnings:

persistent/cPickleCache.c:521:43: warning: comparison of integers of different signs: 'Py_ssize_t' (aka 'long') and 'unsigned long' [-Wsign-compare]
                (v->ob_type->tp_basicsize >= sizeof(cPersistentObject))
                 ~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~
persistent/cPickleCache.c:709:39: warning: comparison of integers of different signs: 'Py_ssize_t' (aka 'long') and 'unsigned long' [-Wsign-compare]
    else if (v->ob_type->tp_basicsize < sizeof(cPersistentObject))

Fixes #69.