Mutual exclusion -- for use with module sched
A mutex has two pieces of state -- a 'locked' bit and a queue.
When the mutex is not locked, the queue is empty.
Otherwise, the queue contains 0 or more (function, argument) pairs
representing functions (or methods) waiting to acquire the lock.
When the mutex is unlocked while the queue is not empty,
the first queue entry is removed and its function(argument) pair called,
implying it now has the lock.
Of course, no multi-threading is implied -- hence the funny interface
for lock, where a function is called once the lock is aquired.