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).
* Replace Parsec with Megaparsec (see #289)
This builds upon PR #289 by @rasendubi
* Revert renaming of parseWithState to parseWithCtx
* Fix doctests
* Update for Megaparsec 5
* Specialize parser to improve performance
* Pretty print errors
* Swap StateT and ParsecT
This is necessary to get the correct backtracking behavior, i.e. discard
state changes if the parsing fails.