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/drop_multi_loop.rs

@@ -29,5 +29,5 @@ fn dropping_loops_does_not_cause_starvation() {
 
     send_signal(TEST_SIGNAL);
 
-    run_with_timeout(&mut rt, signal.into_future());
+    let _ = run_with_timeout(&mut rt, signal.into_future());
 }