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/asm/format.cpp

@@ -5,11 +5,12 @@
 #include <algorithm>
 #include <inttypes.h>
 #include <math.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <string>
 
-#include "asm/fixpoint.hpp"
 #include "asm/main.hpp" // options
 #include "asm/warning.hpp"