make Queue::reachable() iterator much more lean

mino/src/queue.rs

@@ -38,8 +38,9 @@ impl<'a, T: Copy> Queue<'a, T> {
     /// Yields the next pieces available to use from the queue. The returned iterator may
     /// be empty, and may yield at most 2 elements.
     pub fn reachable(&self) -> impl Iterator<Item = T> {
-        let front = self.next.get(0).copied();
-        [front, self.hold].into_iter().flatten()
+        let mut hold = self.hold;
+        let mut front = self.next.get(0).copied();
+        core::iter::from_fn(move || hold.take().or_else(|| front.take()))
     }
 }