Implement thread module

[mblenczewski]

• write the thread control block struct
• write the thread struct
• write a priority based hard real-time scheduler
• write methods to schedule threads in, and for threads to yield (schedule themselves out)
• mock the scheduler (for easier testing)
• write tests to ensure that mutliple threads can run at the same time
• write tests to ensure that higher priority threads are not interrupted by lower priority threads