Adding QL Comments

Checkpoint -- still doesn't pass all unit tests, but it's getting there.

Along the way, gave up on QLPlainTextStage: I just haven't found it all
that relevant in practice, and it's a pain in the tuchus to parse.

Comments require forbidding Names from containing consecutive slashes,
but that seems reasonable.
Comments are now fully functional

Took a good deal of messing with the parser, but I think you can now put
comments in all reasonable positions.