equal visitor useless, since two equals formulae will now
share the same address.
* src/ltlast/multop.hh (add_sorted): New function.
(paircmp): New comparison functor.
(map): Use paircmp, we want to compare the vectors' contents,
not their addresses.
* src/ltlast/multop.cc (add_sorted): New function.
(add): Use it.
* src/ltltest/equals.cc, src/ltltest/tostring.cc: Compare
pointers instead of calling equal.
* src/ltlvisit/equals.cc, src/ltlvisit/equals.hh: Delete.
* src/ltlvisit/Makefile.am (libltlvisit_la_SOURCES): Remove
equals.cc and equals.hh.
* wrap/spot.i: Do not include equals.hh.
