basic + fundamental: Clean up includes

Split out of #37344.

src/basic/iterator.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include "forward.h"
+
 /* Ideally the Iterator would be an opaque struct, but it is instantiated
  * by hashmap users, so the definition has to be here. Do not use its fields
  * directly. */