ltlfilt: add option to filter by implication or equivalence

* src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh: Add a
implication() option.
* src/bin/ltlfilt.cc: Add options --implied-by, --imply, and
--equivalent-to.

src/ltlvisit/simplify.cc

@@ -4260,6 +4260,12 @@ namespace spot
       return cache_->lcc.equal(f, g);
     }
 
+    bool
+    ltl_simplifier::implication(const formula* f, const formula* g)
+    {
+      return cache_->lcc.contained(f, g);
+    }
+
     bdd
     ltl_simplifier::as_bdd(const formula* f)
     {