Add methods to split off array chunks from slices

[JarredAllen]

I implemented the slice methods described in my ACP rust-lang/libs-team#543:

impl<T> [T] {
    pub const fn split_off_first_chunk<'a, const N: usize>(self: &mut &'a Self) -> Option<&'a [T; N]>;
    pub const fn split_off_first_chunk_mut<'a, const N: usize>(self: &mut &'a mut Self) -> Option<&'a mut [T; N]>;
    pub const fn split_off_last_chunk<'a, const N: usize>(self: &mut &'a Self) -> Option<&'a [T; N]>;
    pub const fn split_off_last_chunk_mut<'a, const N: usize>(self: &mut &'a mut Self) -> Option<&'a mut [T; N]>;
}

The only difference from my original ACP post was that I put the first/last before chunk in the name instead of after it (the ACP has also been updated to match these names). My default intuition when I wrote that ACP was to have the names all appear next to each other alphabetically, but as I was implementing it I checked for similar-looking functions, and I found that split_first_chunk and related methods (which are basically these methods but without the in-place modification of self) do the other order and I decided it would be better to match how the other functions are named.

This is my first PR into rust itself, so I did my best to figure out the norms (both for the process around making a PR and the code I put in the PR) and follow them, but I probably got some stuff wrong so I'm sorry in advance for whatever I got wrong.

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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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