Note that this is semantically different, and will produce different
results. In particular, the sort is now stable, i.e. account names with
the same balance will not have their order reversed relative to each
other. In this context there doesn't seem to be a reason to prefer one
order to the other, so I'm making this change.
Exceptions are for dealing with the pamount field, which is really just
dealing with an unnormalised list of amounts.
This creates an API for dealing with MixedAmount, so we never have to
access the internals outside of Hledger.Data.Amount.
Also remove a comment, since it looks like #1207 has been resolved.
