Add some basic type parser tests

[kennknowles]

Hello,

In order to more easily isolate parsing failures when editing the grammar, I thought I would begin a test suite just for the type parser. This is just a skeleton but I hope it is useful anyhow.

Notes and limitations:

• I did not find intentional equality on types in the codebase, so I used toString() which is obviously fragile and must be replaced in the long term.
• I just mimicked the parser creation from compile.js but am not entirely sure what that's about. Is it because it would take a layer of indirection from the generated parser to encapsulate the extra lexing logic?

• I would love to express all of these tests as the property identity == parse . pretty in a QuickCheck-like framework (like I did here).

  • We would need to choose/write such a framework. (I've had some success with killdream/claire but could see wanting to avoid a LiveScript dependency and/or create something that just directly mirrors QuickCheck and SmallCheck's concepts.)
  • And create a generator for an arbitrary type, which is easy and almost always worthwhile.
  • This property, along with just a few regression tests, is great for ensuring the health of a parser without having mountains of test cases to modify whenever you make a change to the language.

  - Kenn

[joneshf]

I was hoping someone more qualified would have looked at this by now, but I'm fine with it. Maybe @puffnfresh, @taku0, or someone can give it a glance. If not I'll merge it.

Until we get a quickcheck implementation in roy like suggested in #21, I don't see a problem in using claire. The source may be written in ls, but what we'd get from npm would be js, so there's no extra dependency for it. Feel free to add it if it makes this easier/better.

[taku0]

It seems fine to me.