Rollup merge of #76120 - LukasKalbertodt:add-as-slice-method-to-array, r=Mark-Simulacrum

Rollup merge of #76120 - LukasKalbertodt:add-as-slice-method-to-array, r=Mark-Simulacrum

Add `[T; N]::as_[mut_]slice`

Part of me trying to populate arrays with a couple of basic useful methods, like slices already have. The ability to add methods to arrays were added in #75212.  Tracking issue: #76118

This adds:

```rust
impl<T, const N: usize> [T; N] {
    pub fn as_slice(&self) -> &[T];
    pub fn as_mut_slice(&mut self) -> &mut [T];
}
```

These methods are like the ones on `std::array::FixedSizeArray` and in the crate `arraytools`.