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/linkdefs.cpp

@@ -2,6 +2,8 @@
 
 #include "linkdefs.hpp"
 
+#include <string>
+
 using namespace std::literals;
 
 // The default values are the most lax, as they are used as-is by RGBASM; only RGBLINK has the full