TypeJoinVisitor doesn't support visit_overloaded()

[JamesGuthrie]

I'm still experimenting with #638, I have made some progress, but seem to have stumbled across a mypy bug with my test case. The type checker fails because the type Overloaded is not handled by the TypeJoinVisitor. How should the Overloaded type be joined with other types?

[JukkaL]

Hmm... I started thinking about this and it's about as easy (or difficult) to implement this as to specify how it would work. I hope to have an implementation ready in a couple of days, but this is actually pretty subtle if we want to deal correctly with Any types, etc.

[JukkaL]

Finally got around to fixing this. Please let me know if the fix works for you.