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/style.cpp

@@ -5,6 +5,7 @@
 
 #include "style.hpp"
 
+#include <stdio.h>
 #include <stdlib.h> // getenv
 #include <string.h>