Port to ghc-8.8

Currently, compiling with base 4.13 results in compilation failures related to MonadFail.

The issue is that MonadParsec does not (and probably never will) require MonadFail, but our lexer primitives are written using a MonadParsec constraint and also use fail quite a bit. A simple fix would be to add a MonadFail constraint to the MonadParser constraint alias, a proper fix would be to remove fail in favor of better error reporting mechanisms provided by megaparsec.