Implement AsFd for RawPool

Makes sense to provide alongside `Write` and `Seek`, and useful for
using shm buffers with other protocols like Pipewire.

`SlotPool` can't provide this, as a safe accessor.

src/shm/raw.rs

@@ -10,7 +10,7 @@ use rustix::{
 use std::{
     fs::File,
     io,
-    os::unix::prelude::{AsFd, OwnedFd},
+    os::unix::prelude::{AsFd, BorrowedFd, OwnedFd},
     sync::Arc,
     time::{SystemTime, UNIX_EPOCH},
 };
@@ -151,6 +151,12 @@ impl RawPool {
     }
 }
 
+impl AsFd for RawPool {
+    fn as_fd(&self) -> BorrowedFd {
+        self.mem_file.as_fd()
+    }
+}
+
 impl io::Write for RawPool {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
         io::Write::write(&mut self.mem_file, buf)