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/src/ctrl_c.rs

@@ -0,0 +1,55 @@
+#[cfg(unix)]
+use crate::unix::Signal as Inner;
+#[cfg(windows)]
+use crate::windows::Event as Inner;
+use crate::IoFuture;
+use futures_core::stream::Stream;
+use futures_util::future::FutureExt;
+use futures_util::try_future::TryFutureExt;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+use tokio_reactor::Handle;
+
+/// Represents a stream which receives "ctrl-c" notifications sent to the process.
+///
+/// In general signals are handled very differently across Unix and Windows, but
+/// this is somewhat cross platform in terms of how it can be handled. A ctrl-c
+/// event to a console process can be represented as a stream for both Windows
+/// and Unix.
+///
+/// Note that there are a number of caveats listening for signals, and you may
+/// wish to read up on the documentation in the `unix` or `windows` module to
+/// take a peek.
+#[must_use = "streams do nothing unless polled"]
+#[derive(Debug)]
+pub struct CtrlC {
+    inner: Inner,
+}
+
+impl CtrlC {
+    /// Creates a new stream which receives "ctrl-c" notifications sent to the
+    /// process.
+    ///
+    /// This function binds to the default reactor.
+    pub fn new() -> IoFuture<Self> {
+        Self::with_handle(&Handle::default())
+    }
+
+    /// Creates a new stream which receives "ctrl-c" notifications sent to the
+    /// process.
+    ///
+    /// This function binds to reactor specified by `handle`.
+    pub fn with_handle(handle: &Handle) -> IoFuture<CtrlC> {
+        Inner::ctrl_c(handle).map_ok(|inner| Self { inner }).boxed()
+    }
+}
+
+impl Stream for CtrlC {
+    type Item = ();
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Pin::new(&mut self.inner)
+            .poll_next(cx)
+            .map(|item| item.map(|_| ()))
+    }
+}