Fix Piece compareTo ordering

This is a fix for the case of Piece.compareTo returning inconsistent ordering.

Specifically if you have (pseudocode):

  Piece p1, p2;
  p1.seen = p2.seen = <some_int>

Then you'd would see with the old code:

  p1.compareTo(p2) == p2.compareTo(p1) == 1

This is wrong and will give headaches to sorting algorithms.  One of those should be -1, they should only be equal when they both return 0.  The new code provides consistent ordering by seen then index.