Make DecodeBase{32,64} always return vector, not string

Base32/base64 are mechanisms for encoding binary data. That they'd
decode to a string is just bizarre. The fact that they'd do that
based on the type of input arguments even more so.

src/util/message.cpp

@@ -36,7 +36,7 @@ MessageVerificationResult MessageVerify(
     }
 
     bool invalid = false;
-    std::vector<unsigned char> signature_bytes = DecodeBase64(signature.c_str(), &invalid);
+    std::vector<unsigned char> signature_bytes = DecodeBase64(signature, &invalid);
     if (invalid) {
         return MessageVerificationResult::ERR_MALFORMED_SIGNATURE;
     }