Add derive Decode for transparent enums

[bsh98]

Summary

Adds support for deriving ssz::Decode on #[ssz(enum_behaviour = "transparent")] enums by attempting to decode all variants in order until one is successful. Returns ssz::DecodeError::NoMatchingVariant if none can be successfully decoded. This is the same approach used by serde for their untagged enum representation.

Motivation

This is useful in situations where we are expecting one of a few distinctly decodable types.

Considerations

In some cases, the ordering of the enum variants will matter and this may not always be obvious to the user. The user must be careful not to include a variant that will "shadow" subsequent variants. It would be ideal to warn/panic in these cases. (Is this possible?)

[michaelsproul]

Sorry for the slow review!

As you noted, this sort of Decode implementation has its downsides. I would be OK with merging this PR anyway, as users can opt-out of deriving Decode if they don't desire this behaviour.

I'm afk at the moment but will review the code more thoroughly tomorrow

[michaelsproul]

Nice, very clean impl. I'll publish a new release with this change shortly!