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/extern/utf8decoder.cpp

@@ -6,6 +6,8 @@
 
 #include "extern/utf8decoder.hpp"
 
+#include <stdint.h>
+
 // clang-format off: vertically align values
 static uint8_t const utf8d[] = {
     // The first part of the table maps bytes to character classes that