* src/graph/graph.hh (new_states): Call reserve().
* src/tgbaalgos/randomgraph.cc: Use tgba_digraph
instead of tgba_string_explicit.

* 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 (trans_iterator): Add operator->() and operator
bool().
* src/tgbaalgos/sccinfo.cc, src/tgbaalgos/sccinfo.hh: New files.
* src/tgbaalgos/Makefile.am: Add them.
* src/tgbatest/ltl2tgba.cc: Use the new dump_scc_info_dot() function.

* 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.

* src/tgba/tgbagraph.hh: New file.
* src/tgba/Makefile.am: Add it.
* src/graph/graph.hh: Add methods needed by tgbagraph.hh.
* src/graphtest/tgbagraph.cc, src/graphtest/tgbagraph.test: New files.
* src/graphtest/Makefile.am: Add them.

* src/graph/graph.hh, src/graph/ngraph.hh: Here.

* src/graph/graph.hh (boxed_label): If State_Data==void, inherit from
std::tuple<> and implement a data() method.
(digraph::state_data): Return by reference.
* src/graphtest/ngraph.cc, src/graphtest/ngraph.test: Test the
case where State_Data implements the spot::state interface.

* src/graph/graph.hh, src/graph/Makefile.am, src/graphtest/graph.cc,
src/graphtest/graph.test, src/graphtest/defs.in,
src/graphtest/Makefile.am: New files.
* src/Makefile.am, configure.ac: Add graph/ and graphtest/.
* README: Mention these directories.