Derive `Copy` and `Hash` for `IntErrorKind`
This PR derives `Copy` and `Hash` for `IntErrorKind` to make it easier to work with. (see #131826)
I think an argument could be made to also derive `PartialOrd` + `Ord` as well given that other error kinds in the std like [`io::ErrorKind`](https://doc.rust-lang.org/src/std/io/error.rs.html#212-428) do this. Granted these seem much less useful for errors.
Fixes https://github.com/rust-lang/rust/issues/131826