differing types for either/maybe

either is not itself a type, but instead Left and Right are types. Similarly, maybe isn't a type but Just and Nothing are. I think Either and Maybe should be types that can either be Left/Right or Maybe/Nothing (like the Fantasy Land versions). This would make it more consistent in terms of situations where you need to type check (such as specs), but also to be able to implement `ILookup` for `core.match` for these types