closure, gram: add missing const

* src/closure.h, src/closure.c, src/gram.h, src/gram.c: Add some missing
const where appropriate.

src/closure.h

@@ -44,7 +44,7 @@ void new_closure (unsigned int n);
    significant).  CLOSURE places there the indices of all items which
    represent units of input that could arrive next.  */
 
-void closure (item_number *items, size_t n);
+void closure (item_number const *items, size_t n);
 
 
 /* Frees ITEMSET, RULESET and internal data.  */