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/ltlvisit/reduce.cc

@@ -22,7 +22,6 @@
 // 02111-1307, USA.
 
 #include "reduce.hh"
-#include "basicreduce.hh"
 #include "ltlast/allnodes.hh"
 #include <cassert>
 
@@ -74,13 +73,6 @@ namespace spot
 	  f2->destroy();
 	  f2 = f1;
 
-	  if (opt & Reduce_Basics)
-	    {
-	      f1 = basic_reduce(f2);
-	      f2->destroy();
-	      f2 = f1;
-	    }
-
 	  f1 = simplifier.simplify(f2);
 	  f2->destroy();
 	  f2 = f1;