[ race ] Make Either be inside the racing primitive

[buzden]

This makes primitives for zipping and for racing looks very symmetric, in the form of C a -> C b -> C (Tambara a b) for particular tambaras of Pair and Either for zipping and racing respectively.

[buzden]

However, I doubt it's worth to be merged because it creates additional info (of the side using Either) and ignores it when it's not used (in <|>, most cases, I think) rather than adds this additional info only for when it's needed (as in #26). Thoughts of constant allocations and droppings tear apart my soul when I think about using Coop again with Arduino (although, the current design is not checked to be optimal according to allocations anyway).

This, by the way, causes different ordering of operations happened in the same moment of time, which is not beautiful, but I think is not really significant in real use.