After a parallel GC, it is possible to have a long list of blocks in
ws->part_list, if we did a lot of work stealing but didn't fill up the
blocks we stole.  These blocks persist until the next load-balanced GC,
which might be a long time, and during every GC we were traversing this
list to find its size.  The fix is to maintain the size all the time, so
we don't have to compute it.