Fixed Setup::SubsumesWithSplit(). Faster Setup::PropagateUnits().

* Setup::SubsumesWithSplit() added the positive clause twice and the
  negative clause not at all.
* Setup::PropagateUnits() stops if no new unit clauses were generated.
Simplified inconsistency management.

Setup::incons_ is now of type Setup::BitMap which overrides operator[]
to resize to the requested element.