This version has optimization for explicit twa, and also for weak and
terminal (depending on whether an accepting run is requested) automata.

* spot/twaalgos/couvreurnew.hh, spot/twaalgos/couvreurnew.cc,
  spot/twaalgos/Makefile.am: New files for the new algorithm.
* spot/twaalgos/emptiness.cc, tests/core/randtgba.cc:
  Register new algorithm.