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/link/section.hpp

@@ -3,9 +3,6 @@
 #ifndef RGBDS_LINK_SECTION_HPP
 #define RGBDS_LINK_SECTION_HPP
 
-// GUIDELINE: external code MUST NOT BE AWARE of the data structure used!
-
-#include <deque>
 #include <memory>
 #include <stdint.h>
 #include <string>
@@ -13,8 +10,6 @@
 
 #include "linkdefs.hpp"
 
-#include "link/main.hpp"
-
 struct FileStackNode;
 struct Section;
 struct Symbol;