Add a method to manually deregister an I/O object

Typically this happens automatically as the `E` in `PollEvented<E>` is an owned
reference (e.g. a `TcpStream`) where dropping that will close the resource,
automatically unregistering it from the event loop. In some situations, however,
this isn't always the case, so the deregistering needs to happen manually.

src/reactor/mod.rs

@@ -435,6 +435,10 @@ impl Inner {
         Ok((sched.readiness.clone(), entry.insert(sched).index()))
     }
 
+    fn deregister_source(&mut self, source: &mio::Evented) -> io::Result<()> {
+        self.io.deregister(source)
+    }
+
     fn drop_source(&mut self, token: usize) {
         debug!("dropping I/O source: {}", token);
         self.io_dispatch.remove(token).unwrap();