add nonpositive/nonnegative predicates/handlers
Rename the file to what I keep expecting it to be
Add ask handlers for Symbol that check the old assumptions

So far, what doesn't work is if you assume one thing in the old assumptions
and a contradictory thing in the new, and then ask the thing you assumed from
the old.  See the added tests (the "2" tests fail).
Merge remote-tracking branch 'mrocklin/nonpositive' into assumptions