signal: Replace ctrl_c with a CtrlC struct (#1273)

* Add a new `CtrlC` struct which will represent a stream of SIGINT
signals on Unix or the CTRL_C event on Windows
* `CtrlC` implements `Stream<Output = ()>` rather than `IoSteam` as
previously

tokio-signal/tests/multi_loop.rs

@@ -22,7 +22,7 @@ fn multi_loop() {
                     let mut rt = CurrentThreadRuntime::new().unwrap();
                     let signal = run_with_timeout(&mut rt, Signal::new(libc::SIGHUP)).unwrap();
                     sender.send(()).unwrap();
-                    run_with_timeout(&mut rt, signal.into_future());
+                    let _ = run_with_timeout(&mut rt, signal.into_future());
                 })
             })
             .collect();