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/asm/charmap.hpp

@@ -4,6 +4,7 @@
 #define RGBDS_ASM_CHARMAP_HPP
 
 #include <optional>
+#include <stddef.h>
 #include <stdint.h>
 #include <string>
 #include <string_view>