Implement Default for some alloc/core iterators

This way one can `mem::take()` them out of structs or #[derive(Default)] on structs containing them.

These changes will be insta-stable.

library/core/src/slice/iter/macros.rs

@@ -393,6 +393,13 @@ macro_rules! iterator {
                 }
             }
         }
+
+        #[stable(feature = "default_iters", since = "CURRENT_RUSTC_VERSION")]
+        impl<T> Default for $name<'_, T> {
+            fn default() -> Self {
+                (& $( $mut_ )? []).into_iter()
+            }
+        }
     }
 }