Stabilize slice_sort_by_cached_key

[scottmcm]

I was going to ask on the tracking issue (#34447), but decided to just send this and hope for an FCP here. The method was added last March by #48639.

Signature: https://doc.rust-lang.org/std/primitive.slice.html#method.sort_by_cached_key

impl [T] {
    pub fn sort_by_cached_key<K, F>(&mut self, f: F)
        where F: FnMut(&T) -> K, K: Ord;
}

That's an identical signature to the existing sort_by_key, so I think the questions are just naming, implementation, and the usual "do we want this?".

The implementation seems to have proven its use in rustc at least, which many uses: https://github.com/rust-lang/rust/search?l=Rust&q=sort_by_cached_key

(I'm asking because it's exactly what I just needed the other day:

    all_positions.sort_by_cached_key(|&n|
        data::CITIES.iter()
            .map(|x| *metric_closure.get_edge(n, x.pos).unwrap())
            .sum::<usize>()
    );

since caching that key is a pretty obviously good idea.)

Closes #34447