tgbagraph: add a merge_transitions() method.
* src/graph/graph.hh: Add some framework to erase transitions, and defrag the resulting transitions_ vector on demand. Also remove the nb_states() and nb_transitions() because num_states() and num_transitions() already exist. * src/graphtest/graph.cc, src/graphtest/ngraph.cc: Adjust to use num_states(). * src/tgba/tgbagraph.hh (merge_transitions): New method. * src/misc/hash.hh: Add a pair_hash class, needed by merge_transitions(). * src/graphtest/tgbagraph.cc, src/graphtest/tgbagraph.test: Add states for transitions removal and merge_transitions().