AK: Add DuplexMemoryStream class.

This class is similar to BufferStream because it is possible to both
read and write to it. However, it differs in the following ways:

  - DuplexMemoryStream keeps a history of 64KiB and discards the rest,
    BufferStream always keeps everything around.

  - DuplexMemoryStream tracks reading and writing seperately, the
    following is valid:

        DuplexMemoryStream stream;
        stream << 42;
        int value;
        stream >> value;

    For BufferStream it would read:

        BufferStream stream;
        stream << 42;
        int value;
        stream.seek(0);
        stream >> value;

In the future I would like to replace all usages of BufferStream with
InputMemoryStream, OutputMemoryStream (doesn't exist yet) and
DuplexMemoryStream. For now I just add DuplexMemoryStream though.

AK/Forward.h

@@ -50,6 +50,7 @@ class Utf32View;
 class Utf8View;
 class InputStream;
 class InputMemoryStream;
+class DuplexMemoryStream;
 
 template<typename T>
 class Span;
@@ -125,6 +126,7 @@ using AK::Bytes;
 using AK::CircularQueue;
 using AK::DebugLogStream;
 using AK::DoublyLinkedList;
+using AK::DuplexMemoryStream;
 using AK::FixedArray;
 using AK::FlyString;
 using AK::Function;