Define variants in a sub-namespace of their enum

[Rendered](https://github.com/sfackler/rfcs/blob/enum-namespace/active/0000-enum-namespacing.md)
It would certainly make the implementation of this RFC in resolve easier if we don't have to worry about a mix of importable and non-importable things in the same module. Has `use`able inherent methods already been covered by an existing RFC? I can write one up if not.