UnknownDecodable for most enums, plus a special case for color.

[seansfkelley]

Stacked on top of #38 cause I'm not sure if you want this change. After the breakage fixed in that PR I thought it would be prudent to future-proof some of the more breakage-prone parts of the type definitions.

This PR extends UnknownDecodable to almost every enumeration. It's a little bit of a nuisance to use so I made a couple editorial decisions:

• it wouldn't be applied to Color, Rarity, Legality or ImageStatus because they shouldn't change much (if ever) and/or they are used a lot and an extra case is annoying
• producedMana would get its own enum (because of this card) to keep the core Color type from getting dumb

I'm open to any changes. In particular, ProducedColor could also be dropped in favor of just String, or I could explicitly add ProducedMana.T instead of it just being a straight clone of Color + UnknownDecodable.

[JacobHearst]

Stacked on top of #38 cause I'm not sure if you want this change. After the breakage fixed in that PR I thought it would be prudent to future-proof some of the more breakage-prone parts of the type definitions.

I think the extra coverage is worth it. I've been wondering about adding a macro to handle this automatically but not sure how folks feel about macro adoption. It would be nice to get rid of the rather silly CaseIterable test suite though

[JacobHearst]

Unit tests are failing because of rate-limiting, looks like my quick-n-dirty rate limiter isn't working quite right but that's something I'll have to figure out later, no reason to block this PR.