serialization: teach serializers variadics

Also add a variadic CDataStream ctor for ease-of-use.

src/streams.h

@@ -112,6 +112,13 @@ public:
         Init(nTypeIn, nVersionIn);
     }
 
+    template <typename... Args>
+    CDataStream(int nTypeIn, int nVersionIn, Args&&... args)
+    {
+        Init(nTypeIn, nVersionIn);
+        ::SerializeMany(*this, nType, nVersion, std::forward<Args>(args)...);
+    }
+
     void Init(int nTypeIn, int nVersionIn)
     {
         nReadPos = 0;