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/readwritefile.cpp

@@ -5,8 +5,9 @@
 
 #include <fs.h>
 
+#include <algorithm>
+#include <cstdio>
 #include <limits>
-#include <stdio.h>
 #include <string>
 #include <utility>