well check it in.  This yields an overall 133% speedup on a "hot" search
for 'python' in my python-dev archive (a word that appears in all but
2 documents).  For those who read the email, turned out it was a
significant speedup to iterate over an IIBTree's items rather than to
materialize the items into an explicit list first.

This is now within 20% of simply doing "IIBucket(the_IIBTree)" (i.e.,
no arithmetic at all), so there's no significant possibility remaining
for speeding the inner score loop.