Implement AsMut<T> and AsRef<T> for !.

[kpreid]

This will allow e.g. &[!] to satisfy &[T] where T: AsRef<str>. It follows the recommendation from the never documentation:

When writing your own traits, ! should have an impl whenever there is an obvious impl which doesn’t panic!.

— https://doc.rust-lang.org/1.97.1/std/primitive.never.html#-and-traits

Tracking issue for never: #35121
(This change has no ACP or tracking issue of its own; I assume it is simple enough that T-libs-api can just accept or reject this PR, and the implementation will be stable when ! is stable.)

@rustbot label -T-libs +T-libs-api +F-never_type