refactor: add most of src/util to iwyu

These files change infrequently, and not much header shuffling is required.

We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.

src/util/message.cpp

@@ -3,16 +3,20 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <hash.h>            // For CHashWriter
-#include <key.h>             // For CKey
-#include <key_io.h>          // For DecodeDestination()
-#include <pubkey.h>          // For CPubKey
-#include <script/standard.h> // For CTxDestination, IsValidDestination(), PKHash
-#include <serialize.h>       // For SER_GETHASH
+#include <hash.h>
+#include <key.h>
+#include <key_io.h>
+#include <pubkey.h>
+#include <script/standard.h>
+#include <serialize.h>
+#include <uint256.h>
 #include <util/message.h>
-#include <util/strencodings.h> // For DecodeBase64()
+#include <util/strencodings.h>
 
+#include <cassert>
+#include <optional>
 #include <string>
+#include <variant>
 #include <vector>
 
 /**