Some kernel code cleanup and docs improvement.

- Moved all config macros to a new ar_config.h header file.
- Added ar_get_millisecond_count() API.
- Remove ar_yield() and ar_periodic_timer() functions in favor of just
  having the handlers call ar_kernel_yield_isr() and
  ar_kernel_periodic_timer_isr() directly.
- Moved ar_kernel_increment_tick_count() and ar_kernel_scheduler(), and
  list add() and remove() method implementation to ar_kernel.cpp from
  ar_thread.cpp.
- Implemented some missing accessor APIs like ar_get_tick_count().
- Added an as-yet-unused ar_thread_sort_by_wakeup() sort predicate.
- Moved docs for ar doxy group to new argon.md Markdown file.
- Improved numerous documentation comments.
- Updated todo list.
- Deleted ar_named_object.cpp source file.