Here's what I've got so far. I think there's probably more gold to be
mined in terms of reducing runtime memory consumption (I don't like the
list_free_deep bit, we should use a context), but functionally it seems
complete. I'm off to dinner again, it's in your court to look over some
more if you want.
(PS: if you want to apply, go ahead, don't forget catversion bump.)
regards, tom lane