Before this patch, every time you cloned a formula, the clone
visitor would recurse into the entire AST to increment the
reference count of all nodes.  When running ltl2tgba_fm on
the formula generated by "LTLcounterLinear.pl 8", approx 27% of
the time was spent in the clone visitor.

After this patch, cloning a formula is just an increment of the
reference count of the top node.  Children are decremented only
when the top node's ref count is decremented to zero.  With this
change, clone() and destroy() become constant time, the
ltl2tgba_fm spend only 0.01% of the time cloning formulae.

* src/ltlast/automatop.cc (~automatop): Decrement children.
(instance): Decrement children if the instance already exists.
* src/ltlast/binop.cc, src/ltlast/multop.cc, src/ltlast/unop.cc:
Likewise.
* src/ltlvisit/clone.cc (clone): Simplify, now we only need to
call ref().
* src/ltlvisit/destroy.cc (destroy): Simplify, now we only need
to call unref().
(destroy_visitor): Remove, no longer needed.