Types for enum variants

[petrochenkov]

This is an issue for postponed RFC 1450 "Types for enum variants" that proposes making enum variants first-class types.

[alexreg]

So what's the plan from here, @petrochenkov?

[Centril]

Closing in favor of not using postponement issues anymore per discussion in #2348 and relabeling the RFC #1450 with postponed.