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/strencodings.h

@@ -13,11 +13,14 @@
 #include <util/string.h>
 
 #include <charconv>
+#include <cstddef>
 #include <cstdint>
-#include <iterator>
 #include <limits>
 #include <optional>
 #include <string>
+#include <string_view>
+#include <system_error>
+#include <type_traits>
 #include <vector>
 
 /** Used by SanitizeString() */