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/derivation.h

@@ -37,6 +37,18 @@ static inline derivation_list derivation_list_new (void)
   return gl_list_create_empty (GL_LINKED_LIST, NULL, NULL, NULL, true);
 }
 
+static inline bool
+derivation_list_next (gl_list_iterator_t *it, derivation **d)
+{
+  const void *p = NULL;
+  bool res = gl_list_iterator_next (it, &p, NULL);
+  if (res)
+    *d = (derivation *) p;
+  else
+    gl_list_iterator_free (it);
+  return res;
+}
+
 void derivation_list_append (derivation_list dl, derivation *d);
 void derivation_list_prepend (derivation_list dl, derivation *d);
 void derivation_list_free (derivation_list dl);