Revert "gen_cxx: don't SWIG if __cast_to_c__"

This reverts commit b2eda28a7ea5567fc4bfacf5b074578b4a3fa6ed.
Revert "gen_foo: make __cast_to_c__ optional"

This reverts commit facdf8a719b6161f68b670bccd14310ebcff5299.
gen_cxx: hide the cast operator if SWIG
gen_cxx: hide the closures from SWIG sight
Regen neubot.hh libneubot_java libneubot_python
Merge branch 'less_complex_swig_strategy' into devel

Less complex swig strategy: rather than masking whole classes
and rather than replacing such classes with new ones, it seems
enough to mask the things that SWIG does not translate in an
easy way, i.e., the automatic cast and the closures.