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.

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