Implement AsFd and bidirectional conversion to/from OwnedFd

In particular, this makes it possible to get a `BorrowedFd` from an
`Inotify`, which allows integration with polling mechanisms that don't
use `RawFd`. This will be needed to work with the next version of
`async-io`.

src/fd_guard.rs

@@ -1,5 +1,9 @@
 use std::{
     ops::Deref,
+    os::fd::{
+        AsFd,
+        BorrowedFd,
+    },
     os::unix::io::{
         AsRawFd,
         FromRawFd,
@@ -77,6 +81,13 @@ impl AsRawFd for FdGuard {
     }
 }
 
+impl AsFd for FdGuard {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        unsafe { BorrowedFd::borrow_raw(self.fd) }
+    }
+}
+
 impl PartialEq for FdGuard {
     fn eq(&self, other: &FdGuard) -> bool {
         self.fd == other.fd

src/inotify.rs

@@ -1,5 +1,10 @@
 use std::{
     io,
+    os::fd::{
+        AsFd,
+        BorrowedFd,
+        OwnedFd,
+    },
     os::unix::io::{
         AsRawFd,
         FromRawFd,
@@ -370,3 +375,22 @@ impl IntoRawFd for Inotify {
         self.fd.fd
     }
 }
+
+impl AsFd for Inotify {
+    #[inline]
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        self.fd.as_fd()
+    }
+}
+
+impl From<Inotify> for OwnedFd {
+    fn from(fd: Inotify) -> OwnedFd {
+        unsafe { OwnedFd::from_raw_fd(fd.into_raw_fd()) }
+    }
+}
+
+impl From<OwnedFd> for Inotify {
+    fn from(fd: OwnedFd) -> Inotify {
+        unsafe { Inotify::from_raw_fd(fd.into_raw_fd()) }
+    }
+}