twa: store property bits as trivals
* spot/twa/twa.hh: Store property bits as trivals.
* NEWS: Mention the change.
* spot/parseaut/parseaut.yy, spot/twaalgos/are_isomorphic.cc,
spot/twaalgos/complete.cc, spot/twaalgos/dot.cc, spot/twaalgos/hoa.cc,
spot/twaalgos/isdet.cc, spot/twaalgos/isunamb.cc, spot/twaalgos/lbtt.cc,
spot/twaalgos/ltl2tgba_fm.cc, spot/twaalgos/postproc.cc,
spot/twaalgos/remfin.cc, spot/twaalgos/strength.cc,
spot/twaalgos/stutter.cc, spot/twaalgos/stutter.hh,
spot/twaalgos/totgba.cc, tests/core/ikwiad.cc,
tests/python/product.ipynb, tests/python/remfin.py: Adjust.
* doc/org/hoa.org, doc/org/tut21.org: Update documentation.