Add a deprecation message for :from<java>.

I'm not sure whether there's a clean way to utilize the deprecation
mechanism we have, which is why I implemented it as it is implemented at
the moment. Fixes very welcome.
Finishing touches on shortname interop constructors.

As of this commit everything seems to be working with shortname
constructors, but the implementation isn't particularly pretty and also
still has heaps of debug methods in it. I'll clean those up and refactor
at least some existing code duplication when fields work.