cex: style changes in state-item

* src/state-item.h, src/state-item.c (state_item): Make the state
const.
(state_item_set): Make it clearer that it works in the state_items
global array.

src/state-item.h

@@ -63,7 +63,7 @@ typedef int state_item_number;
 
 typedef struct
 {
-  state *state;
+  const state *state;
   item_number *item;
   bitset lookahead;
 } state_item;