Switch from YAPPS to Parsley

[eevee]

Parsley allows for a more complex grammar without needing hacks like _rewind. It also allows for moving #{} interpolation into the expression grammar, which fixes a few edge cases.

I have a branch already that does most of the work.

Unfortunately it's a bit slower, though I did some work to speed it up. Not yet clear how much of a difference this will make in practice.