Port the es5 parser to megaparsec

[achudnov]

Megaparsec (https://github.com/mrkkrp/megaparsec) is a fork of parsec that has a much better interface (compatible with base and mtl) than the original, together with an extensive test-suite. Porting the parser should be simple, although we might need to fix a few things to make it compile/pass the tests.

[philipnilsson]

I believe megaparsec was written for GHC7.8 and above, which I do not use for instance, and of course other potential users as well, so in this case I'd say it might not be worth it yet. I like this as a long term goal.

[achudnov]

They've just ported it to GHC 7.6 (mrkkrp/megaparsec#46). Is that what you are using, Philip? Anyway, I'm not going to work on this right away and I'll make sure to maintain backwards compatibility.

[philipnilsson]

Cool. 7.6 has been around for a pretty long time, I'd be fine with that.