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/gfx/color_set.cpp

@@ -3,6 +3,10 @@
 #include "gfx/color_set.hpp"
 
 #include <algorithm>
+#include <iterator>
+#include <stdint.h>
+#include <stdlib.h>
+#include <utility>
 
 #include "helpers.hpp"