spot::acc_cond::mark_t is implemented as a bit vector using a single
unsigned, and implicit conversions between mark_t and unsigned may be
confusing. We try to use the proper interface.

* bin/autfilt.cc, bin/ltlsynt.cc, spot/kripke/kripke.cc,
  spot/misc/game.hh, spot/parseaut/parseaut.yy, spot/priv/accmap.hh,
  spot/ta/ta.cc, spot/ta/taexplicit.cc, spot/ta/taproduct.cc,
  spot/taalgos/emptinessta.cc, spot/taalgos/tgba2ta.cc, spot/twa/acc.cc,
  spot/twa/acc.hh, spot/twa/taatgba.cc, spot/twa/taatgba.hh,
  spot/twa/twagraph.hh, spot/twaalgos/alternation.cc,
  spot/twaalgos/cleanacc.cc, spot/twaalgos/cobuchi.cc,
  spot/twaalgos/complete.cc, spot/twaalgos/couvreurnew.cc,
  spot/twaalgos/degen.cc, spot/twaalgos/dot.cc,
  spot/twaalgos/dtwasat.cc, spot/twaalgos/dualize.cc,
  spot/twaalgos/emptiness.cc, spot/twaalgos/gtec/ce.cc,
  spot/twaalgos/gtec/gtec.cc, spot/twaalgos/gtec/sccstack.cc,
  spot/twaalgos/gv04.cc, spot/twaalgos/hoa.cc, spot/twaalgos/lbtt.cc,
  spot/twaa...