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/bip32.cpp

@@ -2,12 +2,15 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <sstream>
-#include <stdio.h>
 #include <tinyformat.h>
 #include <util/bip32.h>
 #include <util/strencodings.h>
 
+#include <algorithm>
+#include <cstdint>
+#include <cstdio>
+#include <sstream>
+
 
 bool ParseHDKeypath(const std::string& keypath_str, std::vector<uint32_t>& keypath)
 {