tokio-util: expose key used in DelayQueue's Expired (#4081)

Signed-off-by: Toby Lawrence <toby@nuclearfurnace.com>

tokio-util/src/time/delay_queue.rs

@@ -176,7 +176,7 @@ pub struct Expired<T> {
 ///
 /// [`DelayQueue`]: struct@DelayQueue
 /// [`DelayQueue::insert`]: method@DelayQueue::insert
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Hash)]
 pub struct Key {
     index: usize,
 }
@@ -921,4 +921,9 @@ impl<T> Expired<T> {
     pub fn deadline(&self) -> Instant {
         self.deadline
     }
+
+    /// Returns the key that the expiration is indexed by.
+    pub fn key(&self) -> Key {
+        self.key
+    }
 }