make argonaut fast

The existing tests are not good, I have no confidence this actually works, but the tests pass and I'm going to be on the road for the next 10 hours, so it'll have to do for now.

The idea is to use Javascript's native encoding of JSON, so there's zero overhead between Purescript / Javascript, and also to use Javascript's native functionality for parsing / stringification.
Can this be `Array.isArray(j)`? I don't want to run into context crap with node-webkit or other stuff. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/instanceof#instanceof_and_multiple_context_(e.g._frames_or_windows)