* src/graph/ngraph.hh (alias_state): New method.
* src/graphtest/ngraph.cc: Use it.

* 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 (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/ngraph.hh: New file.
* src/graph/Makefile.am: Add it.
* src/graphtest/ngraph.cc, src/graphtest/ngraph.test: New files.
* src/graphtest/Makefile.am: Add them