Instead, manage all reference counting from ltl::formula.
It ridance of virtual calls to clone() and destroy() easily compensate
the extra test in destroy() to not delete constant nodes.
* src/ltlast/refformula.cc, src/ltlast/refformula.hh: Delete.
* src/ltlast/Makefile.am, wrap/python/spot.i: Adjust.
* src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/binop.cc, src/ltlast/binop.hh, src/ltlast/bunop.cc,
src/ltlast/bunop.hh, src/ltlast/formula.cc, src/ltlast/formula.hh,
src/ltlast/multop.cc, src/ltlast/multop.hh, src/ltlast/unop.cc,
src/ltlast/unop.hh: Ajust the reference counting code.
8d947a8782