Update the sub-stream classes to all inherit from `NonDisposingStream`.
This allows them to correctly implement the `Dispose` pattern, and delegate
the actual disposal to `NonDisposingStream`.
In doing so, we need to remove some redundant overrides from
`NonDisposingStream`, otherwise `BufferedSubStream` would use the
overrides inherited from `NonDisposingStream` instead of the ones
inherited from `Stream` (i.e. delegate `ReadByte` to `Read`).
7z archives may require alternating reads from multiple substreams so it is important to seek before reading from the underlying stream. To keep performance at an acceptable level it is necessary to perform buffering because seeking on every single one-byte-read will destroy performance.
