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/time.h

@@ -9,7 +9,7 @@
 #include <compat.h>
 
 #include <chrono>
-#include <stdint.h>
+#include <cstdint>
 #include <string>
 
 using namespace std::chrono_literals;