Implement From<&'a [T; N]> for Cow<'a, [T]>

[zachs18]

(see also: #116890)

Currently, converting an array reference to a Cow<[T]> requires either slicing and reborrowing (Cow::from(&arr_ref[..])), or explicitly constructing a Cow::Borrowed, sometimes with a type annotation (playground link of some conversions that do and do not compile).

Note that this does not conflict with a possible future impl From<&'a U> for Cow<'_, U>, but could cause inference errors if coexisting with such an impl (where U = [T; N], Cow::from(array_ref) would be ambiguous if not otherwise constrained).

Needs FCP since this is an insta-stable impl.

@rustbot label +T-libs-api

r? @ghost

[scottmcm]

Note that inference regressions for things like Cow::from are tolerable, so you don't necessarily need a fresh test for it.

You could consider something like a top-10k-crates check-only crater run to see whether it's majorly impactful. Inference breaks from extra impls like this default to being allowed, actually -- otherwise we could hardly add anything -- and would only be blocked if the issues are particularly bad and particularly common. (The classic example being something like breaking simple code using literals.)

[zachs18]

Closing in favor of #117342 (which I hadn't seen at the time oops)