Remove T: Debug bound on mpsc Debug impls (#866)

Following from https://github.com/tokio-rs/tokio/pull/865, this PR
removes `#[derive(Debug)]` on `mpsc` sender and receiver types in favor
of explicit `impl fmt::Debug` blocks that don't have a `T: fmt::Debug`
bound.

tokio-sync/src/mpsc/unbounded.rs

@@ -9,7 +9,6 @@ use std::fmt;
 ///
 /// Instances are created by the
 /// [`unbounded_channel`](fn.unbounded_channel.html) function.
-#[derive(Debug)]
 pub struct UnboundedSender<T> {
     chan: chan::Tx<T, Semaphore>,
 }
@@ -20,16 +19,31 @@ impl<T> Clone for UnboundedSender<T> {
     }
 }
 
+impl<T> fmt::Debug for UnboundedSender<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("UnboundedSender")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
 /// Receive values from the associated `UnboundedSender`.
 ///
 /// Instances are created by the
 /// [`unbounded_channel`](fn.unbounded_channel.html) function.
-#[derive(Debug)]
 pub struct UnboundedReceiver<T> {
     /// The channel receiver
     chan: chan::Rx<T, Semaphore>,
 }
 
+impl<T> fmt::Debug for UnboundedReceiver<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("UnboundedReceiver")
+            .field("chan", &self.chan)
+            .finish()
+    }
+}
+
 /// Error returned by the `UnboundedSender`.
 #[derive(Debug)]
 pub struct UnboundedSendError(());