Replace the home-grown parser with one based on rply

**WIP**, please comment/add tests/test on real-world code and feedback.

Fixes #54.
Fixes #126 (more tests would be appreciated).

Adds syntax for quoting (`'foo`), quasiquoting (``` `foo```), unquoting (`~foo`), and unquote-splicing (`~@foo`) (the new syntax needs tests)

Adds a raw string syntax (tests needed).

I think that's about it, which means that all in all the changes are pretty conservative...