* 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.
Make these modules' names more like the heavily-used types they
define (CliOpts, UIOpts, WebOpts). This is consistent with
RawOptions and ReportOptions, and helps with code navigation.
