add commented-out, failing example for cons with references.

rust/pointers.rs

        }
    }
    // pub fn cons_with_ref<A>(x: A, xs: &List<A>) -> List<A> {
    //     match *xs {
    //         Nil => Cons(x, box Nil),
    //         _ => Cons(x, box *xs)
    //                       // ^    "cannot move out of dereference of `&`-pointer"
    //     }
    // }
    pub fn run() {
        println!("\nlist_with_box:");
        println!("l = {}", l);
        println!("cons(3, l) = {:?}", cons(3, l));
        // println!("cons_with_ref(3, &l) = {:?}", cons_with_ref(3, &l));
        //                                                        // ^    "use of moved value: `l`"
    }
}