code: List<Integer> segmentSizes() {
      if (head == null) return Collections.emptyList();
      List<Integer> result = new ArrayList<>();
      result.add(head.limit - head.pos);
      for (Segment s = head.next; s != head; s = s.next) {
        result.add(s.limit - s.pos);
      }
      return result;
    }
docs: For testing. This returns the sizes of the segments in this buffer.