rather than as a postprocessing step. (#1638)
This allows us to have a uniform procedure for balancing transactions,
whether they are normal transactions or forecast transactions, including
dealing with balance assignments, balance assertions, and auto postings.
style amounts according to that argument. journalAddForecast and
journalTransform now return an Either String Journal.
This improves efficiency, as we no longer have to restyle all amounts in
the journal after generating auto postings or periodic transactions.
Changing the return type of journalAddForecast and journalTransform
reduces partiality.
To get the previous behaviour for modifyTransaction, use modifyTransaction mempty.
