Add lvar mutator

This branch adds a mutator for the lvar node. At the moment the only simple mutation is to emit nil.

@mbj when you get a chance I would love if you could review this and let me know if I'm missing anything. In particular I wasn't really certain about the spec layout and whether I should be following the existing spec layout or just write everything using ROM-style specs (which I would gladly do, I just didn't want to change things without your input).