feat(chain): make output of SpkTxOutIndex::unused_spks cloneable

This allows us to pass this to chain sources without calling
`Iterator::collect` first.

crates/chain/src/spk_txout_index.rs

@@ -215,7 +215,7 @@ impl<I: Clone + Ord> SpkTxOutIndex<I> {
     /// let unused_change_spks =
     ///     txout_index.unused_spks((change_index, u32::MIN)..(change_index, u32::MAX));
     /// ```
-    pub fn unused_spks<R>(&self, range: R) -> impl DoubleEndedIterator<Item = (&I, &Script)>
+    pub fn unused_spks<R>(&self, range: R) -> impl DoubleEndedIterator<Item = (&I, &Script)> + Clone
     where
         R: RangeBounds<I>,
     {