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/init.cpp

@@ -72,6 +72,7 @@
 #include <Kernel/Tasks/SyncTask.h>
 #include <Kernel/Time/TimeManagement.h>
 #include <Kernel/VM/MemoryManager.h>
+#include <Kernel/WorkQueue.h>
 #include <Kernel/kstdio.h>
 
 // Defined in the linker script
@@ -189,6 +190,8 @@ extern "C" UNMAP_AFTER_INIT [[noreturn]] void init()
     Process::initialize();
     Scheduler::initialize();
 
+    WorkQueue::initialize();
+
     {
         RefPtr<Thread> init_stage2_thread;
         Process::create_kernel_process(init_stage2_thread, "init_stage2", init_stage2, nullptr);