Define variants in a sub-namespace of their enum

[Rendered](https://github.com/sfackler/rfcs/blob/enum-namespace/active/0000-enum-namespacing.md)
@pcwalton do you know what the story is with the future of `#[feature(globs)]`?

The last I heard is that put-use globs are the real troublemakers, but that the problems seem fixable.