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Add skeletal PointXYComparator

This will be used by TetrisPiece.equals. This method will sort each
incoming Point array in order to get the points in a predictable order
before testing each pair for equality. In order to do this, and make
this more efficient than the naive approach of comparing each point to
each other point, we need to sort in O(n log n) time using Arrays.sort
with this comparator.
Implement PointXYComparator
Implement TetrisPiece.equals(Piece)

Not sure why this doesn't override Object.equals(Object), but OK.
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