Get rid of all dynamic_cast<>s while working on LTL formulae.

They are too slow.

* src/ltlast/formula.hh (opkind, kind, kind_): Use an enum
to indicate the actual kind of the formula.  This way we can
check the kind of a formula without relying on dynamic_cast.
* src/ltlast/atomic_prop.cc, src/ltlast/automatop.cc,
src/ltlast/binop.cc, src/ltlast/bunop.cc, src/ltlast/constant.cc,
src/ltlast/multop.cc, src/ltlast/refformula.cc,
src/ltlast/refformula.hh, src/ltlast/unop.cc: Adjust constructors.
* src/ltlvisit/basicreduce.cc, src/ltlvisit/mark.cc,
src/ltlvisit/reduce.cc, src/ltlvisit/syntimpl.cc,
src/ltlvisit/tostring.cc: Replace all dynamic_cast by a
call to kind() followed by a static_cast.

src/ltlast/formula.hh

@@ -71,11 +71,20 @@ namespace spot
     class formula
     {
     public:
-      formula() : count_(max_count++)
+      /// Kind of a sub-formula
+      enum opkind { Constant,
+		    AtomicProp,
+		    UnOp,
+		    BinOp,
+		    MultOp,
+		    BUnOp,
+		    AutomatOp };
+
+      formula(opkind k) : count_(max_count++), kind_(k)
       {
 	// If the counter of formulae ever loops, we want to skip the
 	// first three values, because they are permanently associated
-	// to constants, and its convenient to have constants smaller
+	// to constants, and it is convenient to have constants smaller
 	// than all other formulae.
 	if (max_count == 0)
 	  max_count = 3;
@@ -100,6 +109,12 @@ namespace spot
       /// Return a canonic representation of the formula
       virtual std::string dump() const = 0;
 
+      /// Return the kind of the top-level operator.
+      opkind kind() const
+      {
+	return kind_;
+      }
+
       ////////////////
       // Properties //
       ////////////////
@@ -279,6 +294,7 @@ namespace spot
     private:
       /// \brief Number of formulae created so far.
       static size_t max_count;
+      opkind kind_;
     };
 
     /// \brief Strict Weak Ordering for <code>const formula*</code>.