(formula_ptr_less_than, formula_ptr_hash): New class.
* src/ltlast/atomic_prop.cc, src/ltlast/binop.cc,
src/ltlast/constant.cc, src/ltlast/formula.cc,
src/ltlast/multop.cc, src/ltlast/unop.cc: Adjust to handle dump_.
* src/ltlvisit/apcollect.cc, src/ltlvisit/apcollect.hh:
Sort the set using formula_ptr_less_than.
* src/ltlvisit/dump.cc: Simplify using ->dump().
* src/tgbaalgos/ltl2tgba_fm.cc: Sort all maps to get deterministic
results.