Do follow-up requests on incomplete type responses

[eboasson]

When Cyclone issues a type lookup request for a type T and all its dependencies, it necessarily includes only those dependencies that is aware of. The types learnt from the response may themselves introduce unknown dependencies.

That means that wait_for_type_resolved_impl_locked may have to issue follow-up requests.

@dpotman Perhaps you can have a look? I'm not sure this is the best way to do it. It does the trick for dynsub on dyntype but that doesn't mean it is good. I'm particularly worried that it could possibly result in a request storm, and that would be quite bad.