Some refactoring and cleanup (#1806)

* Use clang-tidy `misc-include-cleaner` for IWYU `#include` cleanup

* Use `std::optional<size_t>` instead of `ssize_t`

* Rename some functions in linkdefs.hpp

* Fix header order

src/verbosity.cpp

@@ -8,6 +8,8 @@
 #include <stdint.h>
 #include <stdio.h>
 
+#include "style.hpp"
+
 static Verbosity verbosity = VERB_NONE;
 
 bool checkVerbosity(Verbosity level) {