This is exactly was I said I was _not_ going to do in the previous commit,
but apparently hacking late at night had me forget the whole reason that
XIRT is being introduced now---unit tests. I'll be emitting a XIR stream
and I need to parse it for convenience in the tests.
So, here's a good start. Next will be some generalizations that are useful
for the tests as well. This is pretty bare, but accomplishes the task.
See docs for more info.
Mike Gerwitz
b348892276