[derive] intrinsic enum variant read and switching

[jordens]

Unclear how to do the equivalent of "impl Default for Enum::Variant".

With the current limitations to unit and newtypes variants is clear what to do.

split from #166

Doable currently with little overhead using strum. See tests/enum.rs.

[jordens]

Closing for now as this would only move the tag around in the namespace (might even be achievable with flatten).