It looks like str() was meant to truncate the dict, but a missing i+=1 meant
that it never actually did. I also changed the format to include a clear
"..." in case we truncate it, to avoid confusion with a non-truncated dict of
the same size.
This also improves test coverage in subtract() and
NumDict.item_with_largest_value().
refs ticket:2891
Unlike set.union(), which returns a new set, DictOfSets.union() modified
the DictOfSets in-place. The name collision bit me when I changed some
code from using DictOfSets to a normal set, and expected that
set.union() would modify the set in-place. Since there was only one user
of DictOfSets.union, I figured it was safer to just get rid of it.
