Kernel: Implement contended, ref-counted resource framework

This is some syntaxic sugar to use a ContendedResource object with
reference counting. This effectively dissociates merely holding a
reference to an object and actually using the object in a way that
requires locking it against concurrent use.

Kernel/Locking/NonnullRefContendedPtr.h

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <Kernel/Locking/RefCountedContended.h>
+
+namespace Kernel {
+
+template<typename T>
+using NonnullRefContendedPtr = NonnullRefPtr<RefCountedContended<T>>;
+
+}