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/link/fstack.cpp

@@ -1,8 +1,14 @@
 #include "link/fstack.hpp"
 
-#include <inttypes.h>
+#include <stdint.h>
+#include <string>
+#include <utility>
+#include <variant>
+#include <vector>
 
 #include "backtrace.hpp"
+#include "helpers.hpp"
+#include "linkdefs.hpp"
 
 #include "link/warning.hpp"