discover: Implement Clone for Change

Implements Clone for discover::Change, if the underlying key and value
both implement clone.

This is convenient for use-cases where a single change needs to be
duplicated, and sent to multiple discover streams.

tower/src/discover/mod.rs

@@ -97,7 +97,7 @@ where
 }
 
 /// A change in the service set.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub enum Change<K, V> {
  /// A new service identified by key `K` was identified.
  Insert(K, V),