This allows implementing traits that require a raw FD on Arc and Box.
Previously, you'd have to add the function to the trait itself:
```rust
trait MyTrait {
fn as_raw_fd(&self) -> RawFd;
}
impl<T: MyTrait> MyTrait for Arc<T> {
fn as_raw_fd(&self) -> RawFd {
(**self).as_raw_fd()
}
}
```
ed1e3512dc