data: Add is_credit() and is_debit() methods to Posting.

The main motivation for this change is to make sure that higher-level
code deals with the fact that self.units.number can be None, and has
an easy way to do so.

I'm not sure all our code is *currently* doing the right thing for this
case, because I'm not sure it will ever actually come up. It's possible
that earlier Beancount plugins fill in decimal amounts for postings
that are originally loaded with self.units.number=None. I'll have to see
later whether this case comes up in reality, and then deal with it if so.
For now the safest strategy seems to be that most code should operate
when self.units.number is None.