enum: support bitfield enums that use associated constants

[tamird]

I think a composition between rustified_enum and constified_enum_module would achieve this, but I believe they're mutually exclusive today.

[tamird]

Actually that composition won't quite do it, since I really want associated constants on the type rather than a module that contains the type and all the variants.

[emilio]

Fixed by #1293, please reopen if not the case.