style: make item_index a truly different type from item_number

See previous commit.

* src/gram.h (item_index): Make it unsigned.
Fix remaiming issues.

src/print.c

@@ -61,7 +61,7 @@ max_length (size_t *width, const char *str)
 static void
 print_core (FILE *out, state *s)
 {
-  item_number *sitems = s->items;
+  item_index *sitems = s->items;
   size_t snritems = s->nitems;
   /* Output all the items of a state, not only its kernel.  */
   if (report_flag & report_itemsets)