Do we still want the `intertype` function?

The intention of `intertype` was to provide a way to dynamically look up the intertype definition for a Julia type at runtime. We don't seem to ever actually do this though, and there are some design problems around this, like how does this integrate with intertype declarations? Also, I don't really think that this would work in all target languages. So maybe we should just drop it for now, and pick it up if we find a use case.