False/True are problematic in Python, and I don't like that the
enum is op::False but the constructor formula::ff().  So let's
just use ff and tt everywhere, and also eword instead of EmptyWord.

* src/ltlast/formula.hh (False, True, EmptyWord, AP, is_false, is_true):
Rename to...
(ff, tt, eword, ap, is_ff, is_tt): ... these.
* iface/ltsmin/ltsmin.cc, src/ltlast/formula.cc,
src/ltlvisit/apcollect.cc, src/ltlvisit/dot.cc, src/ltlvisit/mark.cc,
src/ltlvisit/mutation.cc, src/ltlvisit/print.cc,
src/ltlvisit/relabel.cc, src/ltlvisit/simpfg.cc,
src/ltlvisit/simplify.cc, src/ltlvisit/snf.cc, src/ltlvisit/unabbrev.cc,
src/twa/acc.cc, src/twa/acc.hh, src/twa/formula2bdd.cc,
src/twaalgos/gtec/gtec.cc, src/twaalgos/hoa.cc, src/twaalgos/ltl2taa.cc,
src/twaalgos/ltl2tgba_fm.cc, src/twaalgos/neverclaim.cc,
src/twaalgos/product.cc, src/twaalgos/remfin.cc, src/twaalgos/safety.cc,
src/tests/parseerr.test, src/tests/utf8.test, wrap/python/spot.py:
Adjust.