Implement AsRawFd/IntoRawFd for RawFd

This is useful to build os abstraction like the nix crate does.
It allows to define functions, which accepts generic arguments
of data structures convertible to RawFd, including RawFd itself.
For example:

  fn write<FD: AsRawFd>(fd: FD, buf: &[u8]) -> Result<usize>

instead of:

  fn write(fd: RawFd, buf: &[u8]) -> Result<usize>
  write(foo.as_raw_fd(), buf);

src/libstd/sys/unix/ext/io.rs

@@ -72,6 +72,13 @@ pub trait IntoRawFd {
     fn into_raw_fd(self) -> RawFd;
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl AsRawFd for RawFd {
+    fn as_raw_fd(&self) -> RawFd {
+        *self
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRawFd for fs::File {
     fn as_raw_fd(&self) -> RawFd {
@@ -84,6 +91,14 @@ impl FromRawFd for fs::File {
         fs::File::from_inner(sys::fs::File::from_inner(fd))
     }
 }
+
+#[stable(feature = "into_raw_os", since = "1.4.0")]
+impl IntoRawFd for RawFd {
+    fn into_raw_fd(self) -> RawFd {
+        self
+    }
+}
+
 #[stable(feature = "into_raw_os", since = "1.4.0")]
 impl IntoRawFd for fs::File {
     fn into_raw_fd(self) -> RawFd {