signal: change unix::Signal to return () instead of signum (#1330)

* This simplifies the API surface by returning () instead of the signal
number that was used during registration. This also more closely mirrors
the cross-platform `CtrlC` event stream API
* This is a **breaking change**

tokio-signal/src/unix.rs

@@ -254,7 +254,6 @@ impl Driver {
 #[derive(Debug)]
 pub struct Signal {
     driver: Driver,
-    signal: c_int,
     rx: Receiver<()>,
 }
 
@@ -321,7 +320,7 @@ impl Signal {
             let (tx, rx) = channel(1);
             globals().register_listener(signal as EventId, tx);
 
-            Ok(Signal { driver, rx, signal })
+            Ok(Signal { driver, rx })
         })
         .boxed()
     }
@@ -332,12 +331,12 @@ impl Signal {
 }
 
 impl Stream for Signal {
-    type Item = c_int;
+    type Item = ();
 
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         let _ = Pin::new(&mut self.driver).poll(cx);
 
-        self.rx.poll_recv(cx).map(|item| item.map(|()| self.signal))
+        self.rx.poll_recv(cx)
     }
 }