rework cardinality interface to use opaque.

I like it better...

src/OpaqueVal.h

@@ -7,6 +7,24 @@
 #include "Val.h"
 #include "digest.h"
 
+class CardinalityCounter;
+
+class CardinalityVal: public OpaqueVal {
+public:
+	CardinalityVal();
+	~CardinalityVal();
+	bool Init(CardinalityCounter*);
+	bool IsValid() const { return valid; };
+	CardinalityCounter* Get() { return c; };
+
+private:
+	bool valid;
+	CardinalityCounter* c;
+
+//	DECLARE_SERIAL(CardinalityVal); Fixme?
+};
+
+
 class HashVal : public OpaqueVal {
 public:
 	virtual bool IsValid() const;