Don't export "private" instances

This fixes #502. I'm not crazy about the method used here, but it seemed the most suitable place to do it in the end.

By "private" I mean the instance is for a class or type that is defined in the same module as the instance, but is not exported, so therefore the instance can't be used outside of the module anyway.
Yep, no problem. I had meant to go back and add the type decls anyway.