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/version.cpp

@@ -2,8 +2,6 @@
 
 #include "version.hpp"
 
-#include <string.h>
-
 #include "helpers.hpp"
 
 // We do not build `make develop` with `-fsanitize=leak` because macOS clang++ does not support it.