* 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().
* src/graph/graph.hh (new_states): New.
* src/tgba/tgbagraph.hh (graph_t): Make it public.
* src/tgbaalgos/simulation.cc: Get read of the acc_compl_automaton
class and replace it by a loop over all states of a tgba_digraph.
Remove some useless data structures.
