New function, performing LTL reduction a la tauriainen.03.a83.
* src/ltltest/equals.cc, src/ltltest/reduc.cc: Add support for
the new reduction.
* src/ltltest/reduc.test: Cut the test in half, and additionally
test the new reduction.
* src/ltltest/reduccmp.test: Run on the new reduction.
* src/ltltest/Makefile.am: Adjust.
* src/tgbatest/ltl2tgba.cc: Add new options to apply the reduction.
* src/tgbatest/spotlbtt.test: Use them.
* src/ltlvisit/contain.cc, src/ltlvisit/contain.hh
(spot::ltl::language_containment_checker): ... in these new files.
* src/ltlvisit/Makefile.am: Adjust.
