Implement "uncompressed cached FS" for speed

[ronaldtse]

While we use DwarFS which allows fast reading performance, the compression mechanism introduces a space-time compromise where the running time gets slower due to compression.

We may want to implement an "pre-decompression" mode where the DwarFS is decompressed before hand.

This is useful in cases where running time is more important than space savings, e.g. in Docker.

[maxirmx]

The core dwarfs has a limitation. It can mount an image embedded in another file, but cannot extract such image.
This dwarfs issue shall be fixes prior to "pre-decompression" mode implementation