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

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

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

    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:

    let array = &mut [1, 2, 3];
    let cells: &[Cell<i32>; 3] = Cell::from_mut(array).as_array_of_cells();