AK: Make IntrusiveRedBlackTree capable of holding non-raw pointers

This is completely based on e4412f1f59
and will allow us to convert some AK::HashMap users in the kernel.

AK/IntrusiveDetails.h

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2021, Ali Mohammad Pur <mpfard@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+namespace AK::Detail {
+
+template<typename T, typename Container>
+struct SubstituteIntrusiveContainerType {
+    using Type = Container;
+};
+
+template<typename T>
+struct SubstituteIntrusiveContainerType<T, NonnullRefPtr<T>> {
+    using Type = RefPtr<T>;
+};
+
+template<typename Container, bool _IsRaw>
+struct SelfReferenceIfNeeded {
+    Container reference = nullptr;
+};
+template<typename Container>
+struct SelfReferenceIfNeeded<Container, true> {
+};
+
+}