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/protocol.h

@@ -349,7 +349,7 @@ public:
         uint64_t nServicesInt = nServices;
         READWRITE(nServicesInt);
         nServices = static_cast<ServiceFlags>(nServicesInt);
-        READWRITE(*static_cast<CService*>(this));
+        READWRITEAS(CService, *this);
     }
 
     // TODO: make private (improves encapsulation)