powerset: rewrite the determinization construction using bitvectors
This helps issue #26 considerably, but I'm not closing it because there are a few places here and there that can be cleaned up. For instance build_state_set in minimize.cc should be rewritten. * src/misc/bitvect.hh (bitvector_array::clear_all): New method. * src/tgbaalgos/powerset.cc (tgba_powerset): Rewrite it. * src/tgbaalgos/powerset.hh (power_map): Simplify.