Several people have asked for a way to check whether a word is
accepted by an automaton, including at least Jonah Romero and Scott
Buckley.  So it's time we have it.

* spot/twa/twa.hh, spot/twa/twa.cc,
spot/twaalgos/word.hh (intersects): Add the new variant.
* spot/twa/fwd.hh: Forward declare twa_word, so that
we can use it in twa.hh.
* spot/twaalgos/forq_contains.cc: Use the new intersection check.
* tests/python/word.ipynb, NEWS: Mention it.
* THANKS: Add Scott Buckley.