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/examples/sighup-example.rs

@@ -20,12 +20,8 @@ mod platform {
 
         // Up until now, we haven't really DONE anything, just prepared
         // our futures, now it's time to actually await the results!
-        while let Some(the_signal) = stream.next().await {
-            println!(
-                "*Got signal {:#x}* I should probably reload my config \
-                 or something",
-                the_signal
-            );
+        while let Some(_) = stream.next().await {
+            println!("*Got signal* I should probably reload my config or something");
         }
     }
 }