AK: Add a forward declaration header

You can now #include <AK/Forward.h> to get most of the AK types as
forward declarations.

Header dependency explosion is one of the main contributors to compile
times at the moment, so this is a step towards smaller include graphs.

AK/Forward.h

@@ -0,0 +1,109 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+namespace AK {
+
+class Bitmap;
+class ByteBuffer;
+class IPv4Address;
+class JsonArray;
+class JsonObject;
+class JsonValue;
+class String;
+class StringBuilder;
+class StringImpl;
+class StringView;
+class URL;
+
+template<typename T>
+class SinglyLinkedList;
+
+template<typename T>
+class DoublyLinkedList;
+
+template<typename T>
+class InlineLinkedList;
+
+template<typename T, int capacity>
+class CircularQueue;
+
+template<typename T>
+class Badge;
+
+template<typename T>
+class FixedArray;
+
+template<typename>
+class Function;
+
+template<typename Out, typename... In>
+class Function<Out(In...)>;
+
+template<typename T>
+class NonnullRefPtr;
+
+template<typename T>
+class NonnullOwnPtr;
+
+template<typename T>
+class RefPtr;
+
+template<typename T>
+class OwnPtr;
+
+template<typename T>
+class WeakPtr;
+
+template<typename T, int inline_capacity = 0>
+class Vector;
+
+}
+
+using AK::Badge;
+using AK::Bitmap;
+using AK::ByteBuffer;
+using AK::CircularQueue;
+using AK::DoublyLinkedList;
+using AK::FixedArray;
+using AK::Function;
+using AK::InlineLinkedList;
+using AK::IPv4Address;
+using AK::JsonArray;
+using AK::JsonObject;
+using AK::JsonValue;
+using AK::NonnullOwnPtr;
+using AK::NonnullRefPtr;
+using AK::OwnPtr;
+using AK::RefPtr;
+using AK::SinglyLinkedList;
+using AK::String;
+using AK::StringBuilder;
+using AK::StringImpl;
+using AK::StringView;
+using AK::URL;
+using AK::Vector;