Add `io::Error::other`
This PR adds a small utility constructor, `io::Error::other`, a shorthand for `io::Error::new(io::ErrorKind::Other, err)`, something I find myself writing often.
For some concrete stats, a quick search on [grep.app](https://grep.app) shows that more than half of the uses of `io::Error::new` use `ErrorKind::Other`:
```
Error::new\((?:std::)?(?:io::)?ErrorKind:: => 3,898 results
Error::new\((?:std::)?(?:io::)?ErrorKind::Other => 2,186 results
```
b742594f4a