`IntErrorKind` should derive `Copy` and `Hash`

When including std errors as part of custom types, I've noticed that [`IntErrorKind`](https://doc.rust-lang.org/std/num/enum.IntErrorKind.html) does not derive the `Copy` or the `Hash` traits, and I think it most probably should, since it would make it easier to propagate and embed it. For example [`IoErrorKind`](https://doc.rust-lang.org/std/io/enum.ErrorKind.html) does in fact derive them.