feat(either_of): Add Either! convenience macro

[DanikVitek]

This PR adds a convenience Either! macro for referring to different EitherOf{N} types.

let a: Either!(A, B, C) = todo!();
let _: EitherOf3<A, B, C> = a;

Specifying a single type parameter just aliases it:

let a: Either!(&str);
let _: &str = a;

Note

TODO: Add an empty branch that aliases the ! (never) type when it gets stabilized,
or possibly add a nightly-only (e.g. nightly_never_type) feature that does this.