Rollup merge of #87944 - oconnor663:as_array_of_cells, r=scottmcm

add Cell::as_array_of_cells, similar to Cell::as_slice_of_cells

I'd like to propose adding `Cell::as_array_of_cells`, as a natural analog to `Cell::as_slice_of_cells`. I don't have a specific use case in mind, other than that supporting slices but not arrays feels like a gap. Do other folks agree with that intuition? Would this addition be substantial enough to need an RFC?

---

Previously, converting `&mut [T; N]` to `&[Cell<T>; N]` looks like this:

```rust
let array = &mut [1, 2, 3];
let cells: &[Cell<i32>; 3] = Cell::from_mut(&mut array[..])
    .as_slice_of_cells()
    .try_into()
    .unwrap();
```

With this new helper method, it looks like this:

```rust
let array = &mut [1, 2, 3];
let cells = Cell::from_mut(array).as_array_of_cells();
```

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