tl: add some implication-based rewritings for "<->", "->", and "xor"
This prevents an exception from being raised if NNF is not performed on Boolean properties and implication-based checks are used. * NEWS: Mention the issue. * spot/tl/simplify.cc, doc/tl/tl.tex: Add some rules. * tests/python/ltlsimple.py: Test them.