They are too slow.

* src/ltlast/formula.hh (opkind, kind, kind_): Use an enum
to indicate the actual kind of the formula.  This way we can
check the kind of a formula without relying on dynamic_cast.
* src/ltlast/atomic_prop.cc, src/ltlast/automatop.cc,
src/ltlast/binop.cc, src/ltlast/bunop.cc, src/ltlast/constant.cc,
src/ltlast/multop.cc, src/ltlast/refformula.cc,
src/ltlast/refformula.hh, src/ltlast/unop.cc: Adjust constructors.
* src/ltlvisit/basicreduce.cc, src/ltlvisit/mark.cc,
src/ltlvisit/reduce.cc, src/ltlvisit/syntimpl.cc,
src/ltlvisit/tostring.cc: Replace all dynamic_cast by a
call to kind() followed by a static_cast.