hashmap: Split out iterator.h

As preparation for #37344, let's split out iterator.h so we can
avoid including the entirety of hashmap.h where it's not needed.

src/basic/iterator.h

@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+/* 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. */
+typedef struct Iterator {
+        const void *next_key; /* expected value of that entry's key pointer */
+        unsigned idx;         /* index of an entry to be iterated next */
+#if ENABLE_DEBUG_HASHMAP
+        unsigned put_count;   /* hashmap's put_count recorded at start of iteration */
+        unsigned rem_count;   /* hashmap's rem_count in previous iteration */
+        unsigned prev_idx;    /* idx in previous iteration */
+#endif
+} Iterator;
+
+#define _IDX_ITERATOR_FIRST (UINT_MAX - 1)
+#define ITERATOR_FIRST ((Iterator) { .idx = _IDX_ITERATOR_FIRST, .next_key = NULL })
+#define ITERATOR_IS_FIRST(i) ((i).idx == _IDX_ITERATOR_FIRST)