Older megaparsec is still supported.
Also cleans up our custom parser types,
and some text (un)packing is done in different places
(possible performance impact).
See the issue and linked mail list discussion. Ambiguity between the
uncleared state, and the "not cleared" --uncleared flag causes confusion
and friction. At this point it seems best to break with Ledger and
past hledger, pick a new name and drop --uncleared to put an end to it.
