lists: fix various issues with the use of gnulib's list

First, we should avoid code such as

    gl_list_iterator_t it = gl_list_iterator (deriv->children);
    derivation *child = NULL;
    while (gl_list_iterator_next (&it, (const void **) &child, NULL))
      {
        derivation_print (child, f);

because of -Wstrict-aliasing (whose job is to catch type-punning
issues).  See https://lists.gnu.org/r/bug-bison/2020-05/msg00039.html.

Rather we need

    gl_list_iterator_t it = gl_list_iterator (deriv->children);
    const void **p = NULL;
    while (gl_list_iterator_next (&it, &p, NULL))
      {
        derivation *child = (derivation *) p;
        derivation_print (child, f);

Second, list iterators actually have destructors.  Even though they
are noop in the case of linked-lists, we should use them.

Let's address both issues with typed wrappers (such as
derivation_list_next) that take care of both issues, and besides allow
to scope the iterators within the loop:

    derivation *child;
    for (gl_list_iterator_t it = gl_list_iterator (deriv->children);
         derivation_list_next (&it, &child);
         )
      {
        derivation_print (child, f);

* src/derivation.h, src/derivation.c (derivation_list_next): New.
Use it where appropriate.
* src/counterexample.c (search_state_list_next): New.
Use it where appropriate.
* src/parse-simulation.h, src/parse-simulation.c
* src/state-item.h (state_item_list_next): New.
Use it where appropriate.

src/parse-simulation.h

@@ -74,6 +74,18 @@
 typedef struct parse_state parse_state;
 typedef gl_list_t parse_state_list;
 
+static inline bool
+parse_state_list_next (gl_list_iterator_t *it, parse_state **ps)
+{
+  const void *p = NULL;
+  bool res = gl_list_iterator_next (it, &p, NULL);
+  if (res)
+    *ps = (parse_state *) p;
+  else
+    gl_list_iterator_free (it);
+  return res;
+}
+
 parse_state *new_parse_state (const state_item *conflict);
 
 size_t parse_state_hasher (const parse_state *ps, size_t max);