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-xml.c

@@ -57,7 +57,7 @@ static struct escape_buf escape_bufs[num_escape_bufs];
 static void
 print_core (FILE *out, int level, 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.  */