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

include/fix/warning.hpp

@@ -3,6 +3,8 @@
 #ifndef RGBDS_FIX_WARNING_HPP
 #define RGBDS_FIX_WARNING_HPP
 
+#include <stdint.h>
+
 #include "diagnostics.hpp"
 
 enum WarningLevel {