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/error.cpp

@@ -5,9 +5,11 @@
 #include <util/error.h>
 
 #include <tinyformat.h>
-#include <util/system.h>
 #include <util/translation.h>
 
+#include <cassert>
+#include <string>
+
 bilingual_str TransactionErrorString(const TransactionError err)
 {
     switch (err) {