This leaves a single state for each in the expr loop. I was trying to
think through ways to have per-token states, but they felt sort of
bulky.
Note this is more verbose: but I think the long-term is going to be that
when we start wanting to add handlers, we're going to need to switch to
different names based on the token found. As a consequence, the parse
state logic will end up diverging a little, and we'll just want to align
towards boilerplate handlers.
Short-term, this opens up a path for saying that each parse node
corresponds to precisely one token in success states, and separates out
what were becoming big handler functions in check.
Jon Ross-Perkins
7c7afc9e32