Define variants in a sub-namespace of their enum

[Rendered](https://github.com/sfackler/rfcs/blob/enum-namespace/active/0000-enum-namespacing.md)
Actually, I believe this would be resolved along with other similar ambiguities. In particular, presumably the variant `Baz` would be considered an "inherent" member of `Bar`, and hence would take priority over "extension" or "trait-defined" members like `Foo::Baz`.