Reimplement basic_reduce()'s rules in ltl_simplifier.

So far I have only checked these rewritings with reduccmp.test.
There are probably a few kinks to iron out.

* src/ltlvisit/simplify.cc: Reimplement most of the basic
rewriting rules, leaving some FIXME comments for dubious ones.
* src/ltlast/multop.cc, src/ltlast/multop.hh: Ignore NULL
pointers in the vector.
* src/ltlvisit/reduce.cc (reduce): Do not call basic_reduce().
* src/ltltest/reduccmp.test: Adjust tests.

src/ltlast/multop.hh

@@ -65,7 +65,8 @@ namespace spot
       /// formulae as argument.  This vector is acquired by the
       /// spot::ltl::multop class, the caller should allocate it with
       /// \c new, but not use it (especially not destroy it) after it
-      /// has been passed to spot::ltl::multop.
+      /// has been passed to spot::ltl::multop.  Inside the vector,
+      /// null pointers are ignored.
       ///
       /// All operators (Or, And, Concat) are associative, and are
       /// automatically inlined.  Or and And are commutative, so their