*-list, *-oset, *-omap: Avoid possible compiler warnings.

*-list, *-oset, *-omap: Avoid possible compiler warnings.

Reported by Marc Nieper-Wißkirchen in
<https://lists.gnu.org/r/bug-gnulib/2020-10/msg00025.html>.

* lib/gl_anylinked_list2.h (gl_linked_iterator,
gl_linked_iterator_from_to): Mark as 'pure'.
(gl_linked_iterator_free): Mark as 'const'.
* lib/gl_anytree_list2.h (gl_tree_size, gl_tree_node_value,
gl_tree_search_from_to, gl_tree_indexof_from_to, gl_tree_iterator,
gl_tree_iterator_from_to, gl_tree_sortedlist_search,
gl_tree_sortedlist_search_from_to, gl_tree_sortedlist_indexof,
gl_tree_sortedlist_indexof_from_to): Mark as 'pure'.
(gl_tree_iterator_free): Mark as 'const'.
* lib/gl_anytree_omap.h (gl_tree_size, gl_tree_iterator): Mark as
'pure'.
(gl_tree_iterator_free): Mark as 'const'.
* lib/gl_anytree_oset.h (gl_tree_size, gl_tree_next_node,
gl_tree_prev_node, gl_tree_iterator): Mark as 'pure'.
(gl_tree_iterator_free): Mark as 'const'.
* lib/gl_anytreehash_list1.h (node_position, compare_by_position,
compare_position_threshold): Mark as 'pure'.
* lib/gl_array_list.c (gl_array_size, gl_array_indexof_from_to,
gl_array_search_from_to, gl_array_iterator, gl_array_iterator_from_to,
gl_array_sortedlist_indexof_from_to, gl_array_sortedlist_indexof,
gl_array_sortedlist_search_from_to, gl_array_sortedlist_search): Mark as
'pure'.
(gl_array_iterator_free): Mark as 'const'.
* lib/gl_array_omap.c (gl_array_size, gl_array_indexof, gl_array_search,
gl_array_search_atleast, gl_array_iterator): Mark as 'pure'.
(gl_array_iterator_free): Mark as 'const'.
* lib/gl_array_oset.c (gl_array_size, gl_array_indexof, gl_array_search,
gl_array_indexof_atleast, gl_array_search_atleast, gl_array_iterator,
gl_array_iterator_atleast): Mark as 'pure'.
(gl_array_iterator_free): Mark as 'const'.
* lib/gl_carray_list.c (gl_carray_size, gl_carray_node_value,
gl_carray_next_node, gl_carray_previous_node, gl_carray_get_at,
gl_carray_indexof_from_to, gl_carray_search_from_to, gl_carray_iterator,
gl_carray_iterator_from_to, gl_carray_sortedlist_indexof_from_to,
gl_carray_sortedlist_indexof, gl_carray_sortedlist_search_from_to,
gl_carray_sortedlist_search): Mark as 'pure'.
(gl_carray_iterator_free): Mark as 'const'.