util: add shrink_to_fit and compact methods to DelayQueue (#4170)

tokio-util/src/time/wheel/mod.rs

@@ -6,6 +6,7 @@ mod stack;
 pub(crate) use self::stack::Stack;
 
 use std::borrow::Borrow;
+use std::fmt::Debug;
 use std::usize;
 
 /// Timing wheel implementation.

tokio-util/src/time/wheel/stack.rs

@@ -1,4 +1,6 @@
 use std::borrow::Borrow;
+use std::cmp::Eq;
+use std::hash::Hash;
 
 /// Abstracts the stack operations needed to track timeouts.
 pub(crate) trait Stack: Default {
@@ -6,7 +8,7 @@ pub(crate) trait Stack: Default {
     type Owned: Borrow<Self::Borrowed>;
 
     /// Borrowed item
-    type Borrowed;
+    type Borrowed: Eq + Hash;
 
     /// Item storage, this allows a slab to be used instead of just the heap
     type Store;