cex: isolate missing API from gl_list

* src/counterexample.c (list_get_end): New.
Use it.
Reduce scopes.

TODO

@@ -1,4 +1,17 @@
 * Bison 3.7
+** Cex
+Don't do this (counterexample.c):
+
+// This is the fastest way to get the tail node from the gl_list API.
+gl_list_node_t
+list_get_end (gl_list_t list)
+{
+  gl_list_node_t sentinel = gl_list_add_last (list, NULL);
+  gl_list_node_t res = gl_list_previous_node (list, sentinel);
+  gl_list_remove_node (list, sentinel);
+  return res;
+}
+
 ** Bistromathic
 - Hitting tab on a line with a syntax error is ugly