The rationale is that this is an internal detail of the cache
    implementation called by Per_dealloc().  It's nice to make it
    fast, and it's good to prevent it from being called with anything
    other than an about-to-be-freed object.

    As a result of it being in the C API, omit tests of refcounts.  We
    would have been in Per_dealloc() otherwise.

    Initialize the percachdel slot of cPersistenceAPIstruct to NULL in
    cPersistence.  In cPickleCache, import the CObject and fill in the
    struct.

Also, reformat many of the comments so that all the text starts to the
right of the /*.

Fiddle the text of the placeholder comment.  I think I understand it
now :-).

Add an XXX comment about the "old" cache API e.g. fullsweep and
reallyfullsweep.

Fix cc_get().  object_from_oid() doesn't ever set an exception.

Remove a Py_FatalError() check from the end of a module init
function.  We don't do that anymore.