misc: fix some down_cast issues
We had new failure on MinGW with GCC believing that some pointer returned by down_cast could be NULL; and the down_cast function was in the global namespace. * spot/misc/casts.hh: Rewrite. * NEWS: Mention the small issues. * tests/core/ikwiad.cc, tests/core/ngraph.cc: Adjust to use spot::down_cast instead of down_cast.