Adds the synchronization primitives (semaphore and count down latch) needed for the rust scheduler.

This count down latch is a little more complex than the C version since it supports multiple waiters. The main difficulties of using multiple waiters in the C version are:

1. There isn't a nice way to synchronize which waiting thread will reset the latch, since it must always be the last waiting thread that resets the latch.
2. If the waiters are modified so that the waiting thread that last takes the lock is the one to reset the latch, it leads to a new issue where the waiting threads could be notified, and one thread resumes and loops all the way back to the wait again before the other threads have a chance to resume.