* spot/twaalgos/toparity.cc, spot/twaalgos/toparity.hh: implement it,
based on last-appearance record (LAR)
* spot/twaalgos/Makefile.am: build it
* NEWS: document it
* python/spot/impl.i: add to python bindings
* tests/Makefile.am, tests/python/toparity.py: test it
