(ltl_to_tgba_fm): Add a new option "containment_checks" to enable
some language containment checks (via emptiness checks) during the
translation.  This first attempt currently only use containment
checks to merge states bisimulating each other.
* src/tgbatest/ltl2tgba.cc: Bind this to option "-c".
* src/tgbatest/spotlbtt.test: Check it.