Add is_x() methods for all EitherN enum variants

[avsaase]

This add is_x() methods on all EitherN enums to more concisely check which future from a SelectN completed first, without accessing the value.

I often use signals for canceling futures "from within" and it's not always needed to access the value of the future, only to know if the signal wait future completed first. This can of course also be achieved using matches!, if-let or let-else, but the methods added in this PR make this a bit more concise.