Hopefully fixes #359.

* spot/misc/trival.hh: Declare a global operator==(trival,trival) that
replace the specialized operator==(bool,trival), and the in class
trival::operator(trival), thanks to the implicit construction from
bool to trival.  Make the repr_t/value_t constructor explicit, are
those are mostly internal to the library and may cause conflicts.
* spot/twa/twa.hh: Adjust to construct trival explicitly.
* python/spot/impl.i: Since Swig/Python does not support global
comparison operators, implement a member version, supporting
only __eq__(trival,bool) as before.
* tests/python/setacc.py: Adjust erroneous code.
* tests/python/trival.py: Add test cases.