* spot/misc/casts.hh: New inline functions and compile-time checks.
* spot/kripke/kripkegraph.hh, spot/ta/taexplicit.cc,
  spot/ta/taproduct.cc, spot/ta/tgtaproduct.cc, spot/taalgos/tgba2ta.cc,
  spot/twa/taatgba.hh, spot/twa/taatgba.cc, spot/twa/twagraph.hh,
  spot/twa/twaproduct.cc, spot/twaalgos/emptiness.cc,
  spot/twaalgos/stutter.cc, spot/ltsmin/ltsmin.cc, tests/core/ikwiad.cc,
  tests/core/ngraph.cc: Remove downcast checks from code.