gram: factor the printing of items and the computation of their rule

There are several places where we need to recover the rule from an
item, let's factor that into item_rule.  We also want to print items
in a nice way: we do it when generating the *output file, but it is
also useful in debug messages.

* src/gram.h, src/gram.c (item_rule, item_print): New.
* src/print.c (print_core): Use them.
* src/state.h, src/state.c: Propagate constness.

src/closure.c

@@ -192,12 +192,12 @@ closure (item_number const *core, size_t n)
 
   /* core is sorted on item index in ritem, which is sorted on rule number.
      Compute itemset with the same sort.  */
+  nitemset = 0;
+  size_t c = 0;
+
   /* A bit index over RULESET. */
   rule_number ruleno;
   bitset_iterator iter;
-
-  nitemset = 0;
-  size_t c = 0;
   BITSET_FOR_EACH (iter, ruleset, ruleno, 0)
     {
       item_number itemno = rules[ruleno].rhs - ritem;