Kernel: Add simplistic work queues

We can't use deferred functions for anything that may require preemption,
such as copying from/to user or accessing the disk. For those purposes
we should use a work queue, which is essentially a kernel thread that
may be preempted or blocked.

Kernel/CMakeLists.txt

@@ -228,6 +228,7 @@ set(KERNEL_SOURCES
     VM/Space.cpp
     VM/VMObject.cpp
     WaitQueue.cpp
+    WorkQueue.cpp
     init.cpp
     kprintf.cpp
 )