* spot/kripke/kripkegraph.hh, spot/misc/hash.hh, spot/twa/taatgba.cc,
spot/twa/twagraph.hh, tests/core/ngraph.cc: Replace subtraction of
pointeur minus nullptr by an explicit cast to size_t.
* spot/twa/acc.hh: Add explicit default copy assignment operator for
rs_pair.