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/lib/ref_counted.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <stdint.h>
+
+#include "debug/debug.h"
+
+template <typename T>
+class RefCounted {
+ public:
+  RefCounted() {}
+  ~RefCounted() { dbgln("RefCounted object destroyed"); }
+  void Adopt() {
+    if (ref_count_ != -1) {
+      panic("Adopting owned ptr");
+    } else {
+      ref_count_ = 1;
+    }
+  }
+
+  void Acquire() {
+    if (ref_count_ == -1) {
+      panic("Acquiring unowned ptr");
+    }
+    ref_count_++;
+  }
+  bool Release() {
+    if (ref_count_ == -1 || ref_count_ == 0) {
+      panic("Releasing unowned ptr");
+    }
+    return (--ref_count_) == 0;
+  }
+
+ private:
+  // FIXME: This should be an atomic type.
+  uint64_t ref_count_ = -1;
+  // Disallow copy and move.
+  RefCounted(RefCounted&) = delete;
+  RefCounted(RefCounted&&) = delete;
+  RefCounted& operator=(RefCounted&) = delete;
+  RefCounted& operator=(RefCounted&&) = delete;
+};