/// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
/// assert_eq!([[1, 2], [3, 4]].join(&[0, 0][..]), [1, 2, 0, 0, 3, 4]);
/// ```
/// assert_eq!([[1, 2], [3, 4]].join(&0), [1, 2, 0, 3, 4]);
/// assert_eq!([[1, 2], [3, 4]].join(&[0, 0][..]), [1, 2, 0, 0, 3, 4]);
/// ```