Groups and Chunks laziness

`GroupBy` and `IntoChunks` are not iterators themselves but implement `IntoIterator` instead.
Then `Groups` and `Chunks` are iterators.
All four are lazy and must be used.

tests/laziness.rs

@@ -72,11 +72,18 @@ must_use_tests! {
         let _ = Panicking.batching(Iterator::next);
     }
     group_by {
+        // GroupBy
         let _ = Panicking.group_by(|x| *x);
+        // Groups
+        let _ = Panicking.group_by(|x| *x).into_iter();
     }
     chunks {
+        // IntoChunks
         let _ = Panicking.chunks(1);
         let _ = Panicking.chunks(2);
+        // Chunks
+        let _ = Panicking.chunks(1).into_iter();
+        let _ = Panicking.chunks(2).into_iter();
     }
     tuple_windows {
         let _ = Panicking.tuple_windows::<(_,)>();