This introduces a new method similar to the previous `delegate`, but with
another closure that allows for handling lookahead tokens from the child
parser.
Admittedly, this isn't exactly what I was going for---a list of arguments
isn't exactly self-documenting, especially with the brevity when the
arguments line up---but this was easy to do and so I'll run with this for
now.
This also modified `delegate` to accept a context, even though it wasn't
necessary, both for consistency with its lookup counterpart and for brevity
with the `into` argument (allowing, in our case, to just pass the name of
the variant, rather than a closure).
I'm not going to handle the actual starting and accepting state stitching
abstraction for now; I'd like to observe future boilerplate more before I
consider the best way to handle it, though I do have some ideas.
DEV-10863
Mike Gerwitz
4cb478a42d