emitter: Parenth stores the value at ->arg_start, not ->left.
emitter: Parenth stores the value at ->arg_start, not ->left.

Found this out by accident when trying to fix #87 (ast should use a chain of
available trees). This caused a crash in sanity.ly at 791, likely because the
new method of re-using trees sticks them at the front, instead of expanding
the back.

Verified that ->arg_start is correct by looking at how emitter evals a parenth
tree when walking it.

This was weird.

jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily,jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily,jesserayadkins/lily