implement first and last for list_with_ref.

rust/pointers.rs

        }
    }
    pub fn first<'a, A>(xs: &'a List<A>) -> Option<&'a A> {
        match *xs {
            Nil => None,
            // note the `ref x` here, the default would again be moving the value
            Cons(ref x, _) => Some(x)
        }
    }
    pub fn last<'a, A>(xs: &'a List<A>) -> Option<&'a A> {
        match *xs {
            Nil => None,
            Cons(ref x, &Nil) => Some(x),
            Cons(_, xs) => last(xs)
        }
    }
    pub fn run() {
        println!("\nlist_with_ref:");
        println!("l = {:?}", l);
        println!("cons(1, l) = {:?}", cons(1, l));
        let l2: &List<int> = &cons(2, l);
        println!("l2 = {:?}", l2);
        println!("first(l2) = {:?}", first(l2));
        println!("last(l2)  = {:?}", last(l2));
    }
}