Implement namespaced enums

[aturon]

Tracking issue for RFC 390.

[aturon]

Nominating. This is a backwards-incompatible change. The intent is not to block 1.0 on this, though, and it has an explicit "atomic" implementation strategy to prevent getting stuck half rolled out.

Suggest: P-backcompat-lang, not 1.0

[pnkfelix]

Assigning P-backcompat-lang, not 1.0 (so if we do not get something for this merged in before 1.0, then we likely will not adopt it at all).