`impl<T: AsRawFd> for {Arc,Box}<T>`

`impl<T: AsRawFd> for {Arc,Box}<T>`

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()
    }
}
```