cex: use better type names

There are too many gl_list_t in there, it's hard to understand what is
going on.  Introduce and use more precise types.  I sure can be wrong
in some places, it's hard to tell without proper tool support.

* src/counterexample.c, src/lssi.c, src/lssi.h, src/parse-simulation.c,
* src/parse-simulation.h, src/state-item.c, src/state-item.h
(si_bfs_node_list, search_state_list, ssb_list, lssi_list)
(state_item_list): New.

src/state-item.h

@@ -69,6 +69,9 @@ typedef struct
   bitset lookahead;
 } state_item;
 
+// A path of state-items.
+typedef gl_list_t state_item_list;
+
 extern bitsetv firsts;
 # define FIRSTS(sym) firsts[(sym) - ntokens]
 
@@ -92,6 +95,7 @@ void state_items_free (void);
 
 bool production_allowed (const state_item *si, const state_item *next);
 
+// Iterating on a state_item_list.
 static inline bool
 state_item_list_next (gl_list_iterator_t *it, state_item **si)
 {