Define variants in a sub-namespace of their enum

[Rendered](https://github.com/sfackler/rfcs/blob/enum-namespace/active/0000-enum-namespacing.md)
@nikomatsakis's [name resolution prototype](https://github.com/nikomatsakis/rust-name-resolution-algorithm) includes support for globs. (From this I would assume that they will be supported, but don't take it from me as official.)