Added T:Send bound to Queue<T> to avoid specialized Drop impl.

Author: pnkfelix

src/libstd/sync/mpsc/spsc_queue.rs

@@ -57,7 +57,7 @@ struct Node<T> {
 /// but it can be safely shared in an Arc if it is guaranteed that there
 /// is only one popper and one pusher touching the queue at any one point in
 /// time.
-pub struct Queue<T> {
+pub struct Queue<T: Send> {
     // consumer fields
     tail: UnsafeCell<*mut Node<T>>, // where to pop from
     tail_prev: AtomicPtr<Node<T>>, // where to pop from