* src/tgbaalgos/powerset.hh (power_map): New structure, allowing
the caller to retrieve the set of original states corresponding to
the set in the deterministic automaton.
(power_set): Adjust prototype to take a power_map instead of the
acc_list.
* src/tgbaalgos/powerset.cc (power_set): Strip all code using
acc_list, and update power_set.
* src/tgbaalgos/minimize.cc (minimize): Rewrite, using an
algorithm similar to the one in the Dax paper to check whether
state of the minimized automaton should be accepting.