Rust in the 6.2 kernel

`Either` is one of the two combinators needed to represent arbitrary arithmetic data types, together with the zero/void (`!`) and one/unit (`()`) atomic types. `Either` represents the capacity for sums (enums). Pairs / tuples are their dual, representing product types (structs). They're both very abstract, but sometimes that's exactly what you want.

It's true that you could just create your own custom enum type instead (which would be isomorphic to some tree of nested `Either` types) but your custom type won't benefit from any functions the standard library might provide for working generically with values which might be "either" one type or another. For example, partitioning a list of `Either<A,B>` into a list of `A` and a list of `B`. I'm not as familiar with what Rust provides for working with `Either` values but in Haskell the `Either` type is a key component of the Arrow abstraction, a more powerful (albeit less popular) alternative to monads, where they are used to represent conditional processing (`ArrowChoice`). It also benefits from ready-made instances of `Bifunctor`, `Bifoldable`, and `Bitraversable`.