Revamp the multop interface to allow some basic optimizations like
not constructing a single-child multop.
* src/ltlast/multop.hh (multop::instance(type)): Remove.
(multop::instance(type, formula*, formula*)): Return a formula*.
(multop::instance(type, vec*)): Make it public and return a formula*.
(multop::add_sorted, multop::add):
* src/ltlast/multop.cc (multop::instance(type, vec*)): Rewrite.
(multop::instance(type)): Delete.
(multop::instance(type, formula*, formula*)): Adjust.
(multop::add_sorted, multop::add): Remove.
* src/ltlvisit/clone.cc (clone_visitor::visit(multop*)) Adjust.
* src/ltlvisit/nenoform.cc
(negative_normal_form_visitor::::visit(multop*)) Adjust.
* src/ltltest/equals.test: Make sure `a & a' and `a' are equals.
* wrap/python/tests/ltlsimple.py: Adjust.
de6314ed74