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.

tests/core/graph.test

@@ -1,7 +1,7 @@
 #!/bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2014 Laboratoire de Recherche et Développement de
-# l'Epita (LRDE).
+# Copyright (C) 2014, 2016 Laboratoire de Recherche et Développement
+# de l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
 #
@@ -71,6 +71,16 @@ digraph {
  2 [label="4"]
  2 -> 1 [label="4"]
 }
+t1: (s0, d0) t2
+t2: (s0, s2) t0
+t3: (s1, s2) t0
+t4: (s2, s1) t0
+s0: t1 t2
+s1: t3 t3
+s2: t4 t4
+d0: #2
+d1: s1
+d2: s2
 digraph {
  0 [label="(2,4)"]
  0 -> 1 [label="(1,3)"]
@@ -84,4 +94,3 @@ digraph {
 EOF
 
 diff stdout expected
-