Reimplement basic_reduce()'s rules in ltl_simplifier.
So far I have only checked these rewritings with reduccmp.test. There are probably a few kinks to iron out. * src/ltlvisit/simplify.cc: Reimplement most of the basic rewriting rules, leaving some FIXME comments for dubious ones. * src/ltlast/multop.cc, src/ltlast/multop.hh: Ignore NULL pointers in the vector. * src/ltlvisit/reduce.cc (reduce): Do not call basic_reduce(). * src/ltltest/reduccmp.test: Adjust tests.