This allows using mutexes in the scheduler code (obviously not in any
scheduling path).

Not yet preemption safe.

This function allows executing a block of code under a lock, with a guarantee
that any exit from the block will release the lock.

Examples:

    with_lock(my_mutex, [&] { a += b; });

    return with_lock(my_spinlock, [&] {
        return my_list.size();
    });

No threading yet, so no real locking