AK: Add a simple Queue<T> class.

The underlying data structure is a singly-linked list of Vector<T>.
We never shift any of the vector contents around, but we batch the memory
allocations into 1000-element segments.

AK/Queue.h

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <AK/OwnPtr.h>
+#include <AK/SinglyLinkedList.h>
+#include <AK/Vector.h>
+
+namespace AK {
+
+template<typename T>
+class Queue {
+public:
+    Queue() { }
+    ~Queue() { }
+
+    int size() const { return m_size; }
+    bool is_empty() const { return m_size == 0; }
+
+    void enqueue(T&& value)
+    {
+        if (m_segments.is_empty() || m_segments.last()->size() >= segment_size)
+            m_segments.append(make<Vector<T, segment_size>>());
+        m_segments.last()->append(move(value));
+        ++m_size;
+    }
+
+    T dequeue()
+    {
+        ASSERT(!is_empty());
+        auto value = move((*m_segments.first())[m_index_into_first++]);
+        if (m_index_into_first == segment_size) {
+            m_segments.take_first();
+            m_index_into_first = 0;
+        }
+        --m_size;
+        return value;
+    }
+
+private:
+    static const int segment_size = 1000;
+
+    SinglyLinkedList<OwnPtr<Vector<T, segment_size>>> m_segments;
+    int m_index_into_first { 0 };
+    int m_size { 0 };
+};
+
+}
+
+using AK::Queue;
+