Include a way to flip sides

[KyNorthstar]

There should be a way to flip which type is on which side.
For example, this would change an Either<String, Int> to an Either<Int, String>. Obviously, the value would switch sides too.

Something like this:

public extension Either {
    var flipped: Either<Right, Left> {
        switch self {
        case .left (let oldLeft ): return .right(oldLeft)
        case .right(let oldRight): return .left(oldRight)
        }
    }
}