Rollup merge of #95198 - clarfonthey:get_chunk, r=scottmcm

Add slice::{split_,}{first,last}_chunk{,_mut}

This adds to the existing tracking issue for `slice::array_chunks` (#74985) under a separate feature, `slice_get_chunk`.

Currently, we have the existing `first`/`last` API for slices:

```rust
impl [T] {
    pub const fn first(&self) -> Option<&T>;
    pub const fn first_mut(&mut self) -> Option<&mut T>;
    pub const fn last(&self) -> Option<&T>;
    pub const fn last_mut(&mut self) -> Option<&mut T>;
    pub const fn split_first(&self) -> Option<(&T, &[T])>;
    pub const fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])>;
    pub const fn split_last(&self) -> Option<(&T, &[T])>;
    pub const fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])>;
}
```

This augments it with a `first_chunk`/`last_chunk` API that allows retrieving multiple elements at once:

```rust
impl [T] {
    pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>;
    pub const fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>;
    pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]>;
    pub const fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>;
    pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>;
    pub const fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>;
    pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>;
    pub const fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>;
}
```

The code is based off of a copy of the existing API, with the documentation and examples properly modified. Currently, the most common way to perform these kinds of lookups with the existing methods is via `slice.as_chunks::<N>().0[0]` or the worse `slice.as_chunks::<N>().0[slice.len() - N]`, which is substantially less readable than `slice.first_chunk::<N>()` or `slice.last_chunk::<N>()`.

ACP: https://github.com/rust-lang/libs-team/issues/69

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