Auto merge of #97437 - jyn514:impl-asrawfd-arc, r=dtolnay

`impl<T: AsRawFd> 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()
    }
}
```

In particular, this leads to lots of "multiple applicable items in scope" errors because you have to disambiguate `MyTrait::as_raw_fd` from `AsRawFd::as_raw_fd` at each call site. In generic contexts, when passing the type to a function that takes `impl AsRawFd` it's also sometimes required to use `T: MyTrait + AsRawFd`, which wouldn't be necessary if I could write `MyTrait: AsRawFd`.

After this PR, the code can be simpler:
```rust
trait MyTrait: AsRawFd {}

impl<T: MyTrait> MyTrait for Arc<T> {}
```

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