Switch from Parsec to Megaparsec

[knrafto]

Megaparsec is more actively maintained, and will give us better error messages

[mmhat]

I guess Language.Bash.Expand is a good place to start. Maybe we fix #17 along the way.

[mmhat]

Another possibility would be happy. I think translating Bashs parse.y to a happy grammar should be pretty straightforward.
@knrafto How do you think about that?

[mmhat]

Would fix #18 as well.

[knrafto]

I doubt Happy would work, because bash's grammar some context-sensitive stuff like heredocs. But even though Parse.y is mostly context-free, I think translating the grammar to Happy would be hard because theres' not really a clean tokenization/lexing phase for bash. It switches between "sublanguages" which are tokenized differently (like arithmetic) so how you need to tokenize next depends on what you've just parsed. That still might be possible with Happy, but I feel like parser combinators are a better fit for dealing with bash's complicated syntax