* 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
- `-fsanitize=undefined` encompasses multiple checks we were specifying
- "detect_leaks=1" for `__asan_default_options` checks for memory leaks
(except for with macOS clang++, which does not support LSan)
- `-fsanitize=float-divide-by-zero` is an extra UBSan check
(and reveals a UB bug to fix with fixed-point `DIV` and `LOG`)