* bench/Makefile.am,
  configure.ac: add new bitstate sub-folder
* bench/bitstate/Makefile.am,
  bench/bitstate/bitstate.cc,
  bench/bitstate/bitstate.hh: implementation here
* spot/mc/Makefile.am,
  spot/mc/bitstate.hh,
  spot/mc/mc.hh,
  tests/ltsmin/modelcheck.cc: remove code from here