AK: Combine SinglyLinkedList and SinglyLinkedListWithCount

Using policy based design `SinglyLinkedList` and
`SinglyLinkedListWithCount` can be combined into one class which takes
a policy to determine how to keep track of the size of the list. The
default policy is to use list iteration to count the items in the list
each time. The `WithCount` form is a different policy which tracks the
size, but comes with the overhead of storing the count and
incrementing/decrementing on each modification.

This model is extensible to have other forms of counting by
implementing only a new policy instead of implementing a totally new
type.

AK/SinglyLinkedListSizePolicy.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+namespace AK {
+
+struct DefaultSizeCalculationPolicy {
+    constexpr void increase_size(auto const&) { }
+
+    constexpr void decrease_size(auto const&) { }
+
+    constexpr void reset() { }
+
+    constexpr size_t size(auto const* head) const
+    {
+        size_t size = 0;
+        for (auto* node = head; node; node = node->next)
+            ++size;
+        return size;
+    }
+};
+
+struct CountingSizeCalculationPolicy {
+    constexpr void increase_size(auto const&) { ++m_size; }
+
+    constexpr void decrease_size(auto const&) { --m_size; }
+
+    constexpr void reset() { m_size = 0; }
+
+    constexpr size_t size(auto const*) const { return m_size; }
+
+private:
+    size_t m_size { 0 };
+};
+
+}
+
+#ifdef USING_AK_GLOBALLY
+using AK::CountingSizeCalculationPolicy;
+using AK::DefaultSizeCalculationPolicy;
+#endif