Create a RefCounted type and use it for Thread.

This should prevent me from actually creating 2 shared ptrs of
a single kernel object with their separate ref counts.

zion/scheduler/scheduler.cpp

@@ -48,7 +48,7 @@ void Scheduler::Preempt() {
     return;
   }
 
-  SharedPtr<Thread> prev = current_thread_;
+  RefPtr<Thread> prev = current_thread_;
   prev->SetState(Thread::RUNNABLE);
   current_thread_ = runnable_threads_.CycleFront(prev);
 
@@ -62,7 +62,7 @@ void Scheduler::Yield() {
   }
   asm volatile("cli");
 
-  SharedPtr<Thread> prev = current_thread_;
+  RefPtr<Thread> prev = current_thread_;
   if (prev == sleep_thread_) {
     if (runnable_threads_.size() == 0) {
       panic("Sleep thread yielded without next.");