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/Forward.h

@@ -76,6 +76,7 @@ class UserOrKernelBuffer;
 class VFS;
 class VMObject;
 class WaitQueue;
+class WorkQueue;
 
 template<typename T>
 class KResultOr;