Auto merge of #87870 - WaffleLapkin:pub_split_at_unchecked, r=dtolnay

Auto merge of #87870 - WaffleLapkin:pub_split_at_unchecked, r=dtolnay

Make `<[T]>::split_at_unchecked` and `<[T]>::split_at_mut_unchecked` public

The methods were originally added in https://github.com/rust-lang/rust/pull/75936 (https://github.com/sdroege/rust/commit/30dc32b10eb53e4a92c61a42062983db58838217), but for some reason as private. Nevertheless, the methods have documentation and even a [tracking issue](https://github.com/rust-lang/rust/issues/76014).

It's very weird to have a tracking issue for private methods and these methods may be useful outside of the standard library. As such, this PR makes the methods public.