graph: replace the existing "alternating" interface
* spot/graph/graph.hh: Use the sign bit of destination state X to designate a universal edge. Store the destinations of such an edge in a separate array, at index ~X. * spot/graph/ngraph.hh, tests/core/graph.cc, tests/core/graph.test, tests/core/ngraph.cc: Adjust test case to the new interface.
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