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`.
10aa3d3f89