Explicit impl Clone for tx to avoid T: Clone (#865)

`#[derive(Clone)]` on a type `struct Foo<T>` adds an impl that requires that
`T: Clone`:

```rust
impl<T: Clone> Clone for Foo<T>
```

which is unfortunate in the case of senders, because we don't want to require
that the items being sent are `Clone` for the channel sender to be `Clone`.
This PR adds an explicit `impl Clone` for the bounded and unbounded sender
types which does not have the `T: Clone` bound.

Note that this is _also_ an issue with `#[derive(Debug)]`, but that one is
harder to work around as `chan::Tx` _also_ has `#[derive(Debug)]`, as does
`chan::Chan`, so we'd have to add explicit impls for all of them to make
progress.

tokio-sync/src/mpsc/unbounded.rs

@@ -9,11 +9,17 @@ use std::fmt;
 ///
 /// Instances are created by the
 /// [`unbounded_channel`](fn.unbounded_channel.html) function.
-#[derive(Debug, Clone)]
+#[derive(Debug)]
 pub struct UnboundedSender<T> {
     chan: chan::Tx<T, Semaphore>,
 }
 
+impl<T> Clone for UnboundedSender<T> {
+    fn clone(&self) -> Self {
+        UnboundedSender { chan: self.chan.clone() }
+    }
+}
+
 /// Receive values from the associated `UnboundedSender`.
 ///
 /// Instances are created by the