* spot/kripke/kripkegraph.hh, spot/priv/bddalloc.hh,
spot/priv/freelist.hh, spot/priv/satcommon.hh, spot/ta/taexplicit.cc
spot/twa/bdddict.cc, spot/twa/twagraph.hh,
spot/twaalgos/alternation.hh, spot/twaalgos/dtwasat.cc,
spot/twaalgos/ltl2taa.cc, spot/twaalgos/stutter.cc,
tests/core/ngraph.cc: Add default constructors, copy constructors, or
remove useless destructors.