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.cpp

@@ -3,17 +3,18 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <span.h>
 #include <util/strencodings.h>
-#include <util/string.h>
-
-#include <tinyformat.h>
 
 #include <algorithm>
 #include <array>
-#include <cstdlib>
+#include <cassert>
 #include <cstring>
 #include <limits>
 #include <optional>
+#include <ostream>
+#include <string>
+#include <vector>
 
 static const std::string CHARS_ALPHA_NUM = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";