priority_queue: implement simple iterator

Remove PriorityQueue::each and replace it with PriorityQueue::iter,
which ultimately calls into vec::VecIterator via PriorityQueueIterator.
Implement iterator::Iterator for PriorityQueueIterator.  Now you should
be able to do:

  extern mod extra;
  let mut pq = extra::priority_queue::PriorityQueue::new();
  pq.push(5);
  pq.push(6);
  pq.push(3);
  for pq.iter().advance |el| {
      println(fmt!("%d", *el));
  }

just like you iterate over vectors, hashmaps, hashsets etc.  Note that
the iteration order is random (as before with PriorityQueue::each), and
_not_ the order you get when you pop() repeatedly.

Add an in-file test to guard this.

Reported-by: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>