Support serialization as another type without casting

This adds a READWRITEAS(type, obj) macro which serializes obj as if it
were casted to (const type&) when const, and to (type&) when non-const.

This makes it usable in serialization code that uses a single
implementation for both serialization and deserializing, which doesn't
know the constness of the object involved.

src/script/script.h

@@ -415,7 +415,7 @@ public:
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action) {
-        READWRITE(static_cast<CScriptBase&>(*this));
+        READWRITEAS(CScriptBase, *this);
     }
 
     CScript& operator+=(const CScript& b)