Rollup merge of #147540 - bjoernager:slice-as-array, r=Amanieu

Stabilise `as_array` in `[_]` and `*const [_]`; stabilise `as_mut_array` in `[_]` and `*mut [_]`.

Tracking issue: rust-lang/rust#133508

Closes: rust-lang/rust#133508

This PR stabilises the `as_array` and `as_mut_array` associated functions from the `core_slice_as_array` feature gate:

```rust
// core::slice

impl<T> [T] {
    pub const fn as_array<const N: usize>(&self) -> Option<&[T; N]>;

    pub const fn as_mut_array<const N: usize>(&mut self) -> Option<&mut [T; N]>;
}

// core::ptr

impl<T> *const [T] {
    pub const fn as_array<const N: usize>(self) -> Option<*const [T; N]>;
}

impl<T> *mut [T] {
    pub const fn as_mut_array<const N: usize>(self) -> Option<*mut [T; N]>;
}
```

It also updates the feature gates and tracking issues for all items associated with the previous `slice_as_array` tracking issue (including these four that are being stabilised).

~~FCP missing.~~

Author: matthiaskrgr