* 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`)
- Fixed-point formulas are implemented using IEEE-754 floating-point
internally, which could give infinity or NaN values whose conversion
to fixed-point integer was platform-dependent.
- Formatting fixed-point $8000_0000 (INT32_MIN, -2147483648) was
not putting the negative sign in front.
- Since we have style rules to include foo.hpp at the top of its
corresponding foo.cpp, this takes any headers included by foo.hpp
as being also guaranteed for foo.cpp.
- Use C-style <foo.h> instead of <cfoo>, since the latter only
guarantees putting symbols in the `std` namespace, which we are
not using for C functions (e.g. `printf` not `std::printf`).
- Remove now-unused `__PRETTY_FUNCTION__` reporting
