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.