Protocol extension, generics and dynamic dispatch confusion

[Slava_Pestov]

@michalka

Since bar is declared in protocol requirements I expect it to be dynamically dispatched to bar properties.

This came up recently:

• Generic implementation doesn't work with concrete types - #6 by Slava_Pestov
• Nested types don't recognize protocol conformance in recursive methods - #6 by Slava_Pestov

Just like in Java and C++, you need to use the "double dispatch" pattern to accomplish this sort of thing: Double dispatch - Wikipedia