Use libc time functions on unix for time module

- Replace chrono-based gmtime/localtime/mktime/strftime/asctime/ctime
  with direct libc calls (gmtime_r, localtime_r, mktime, strftime)
  on unix, keeping chrono as fallback for non-unix platforms
- Add checked_tm_from_struct_time for struct_time field validation
- Accept None in gmtime/localtime/ctime (OptionalArg<Option<>>)
- Fix StructTimeData field order: tm_zone before tm_gmtoff
- Fix PyStructTime slot_new to accept optional dict argument

Author: youknowone