* src/ltlast/unop.hh, src/ltlast/unop.cc: Introduce Closure and
NegClosure operators.
* src/ltlparse/ltlparse.yy: Recognize {foo} as a Closure.
* src/ltlvisit/mark.cc: Consider NegClosure as a marked operator.
* src/tgbaalgos/ltl2tgba_fm.cc (ratexp_trad_visitor): Add option to
select whether the empty_word should act like true (for {SERE}
and {!SERE}) or false (for {SERE}<>->Exp or {SERE}[]->Exp).
(ltl_trad_visitor): Translate Closure and NegClosure.
* src/tgbatest/ltl2tgba.test: Add more tests.
* src/ltlvisit/basicreduce.cc, src/ltlvisit/consterm.cc,
src/ltlvisit/nenoform.cc, src/ltlvisit/reduce.cc,
src/ltlvisit/syntimpl.cc, src/ltlvisit/tostring.cc,
src/ltlvisit/tunabbrev.cc, src/tgba/formula2bdd.cc,
src/tgbaalgos/eltl2tgba_lacim.cc, src/tgbaalgos/ltl2tgba_lacim.cc,
src/tgbaalgos/ltl2taa.cc: Straightforward update to support or
assert on these new operators.