x and xx operators fail when given negatives "-" x -1 should evaluate to an empty string and 0 xx -1 should
evaluate to an empty list.  I have hacked pugs/src/Pugs/Prim.hs to
correctly handle negatives, but I don't know Haskell very well and am
not familiar with the layout of Pugs, so I may have written bad code
in a bad place.  I have also added tests to pugs/t/operators/repeat.t
to test for correct handling of negative and zero values.

Hopefully the patch will make it through to the list.
