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/moneystr.cpp

@@ -10,6 +10,7 @@
 #include <util/strencodings.h>
 #include <util/string.h>
 
+#include <cstdint>
 #include <optional>
 
 std::string FormatMoney(const CAmount n)