Make WitnessUnknown members private

Make sure that nothing else can change WitnessUnknown's data members by
making them private. Also change the program to use a vector rather than
C-style array.

src/util/message.cpp

@@ -47,7 +47,7 @@ MessageVerificationResult MessageVerify(
         return MessageVerificationResult::ERR_PUBKEY_NOT_RECOVERED;
     }
 
-    if (!(CTxDestination(PKHash(pubkey)) == destination)) {
+    if (!(PKHash(pubkey) == *std::get_if<PKHash>(&destination))) {
         return MessageVerificationResult::ERR_NOT_SIGNED;
     }