Switch to more modern parsing/pretty-printing libraries #27

You guys currently depend on parsec and pretty for parsing and pretty-printing respectively. Assuming that you are not doing this purely as a way of minimizing your dependency footprint (since pretty at least is a boot package that ships with GHC), I would recommend that you migrate to more modern parsing/pretty-printing libraries. These can improve parsing speed, parser error message readability, and code reusability.

For parsers, there are two main options, depending on what matters to you:

• If speed is important, then you should use the parsers library to write a typeclass-generic parser, which can then be instantiated with a concrete parser type from trifecta when debugging (trifecta is slow but has good error messages) or with a parser type from attoparsec in production (attoparsec is fast but has bad error messages).
• If nice error messages are important and you want to migrate from your parsec parser easily, but speed is not particularly important, megaparsec is a good option. In the version 6 and above of megaparsec, primitives are provided that make it as fast as attoparsec.

For pretty-printers, my main recommendation is prettyprinter, which is a pretty comprehensive replacement for all the pre-existing pretty-printing libraries in Haskell. It supports terminal output with ANSI colors, HTML output, it doesn't have name clashes with common operators, and it uses Text for performance.