feat: add unsquashfs util to Squashfs

[lovesegfault]

This is mostly a copy from backhand-cli's unsquashfs, but with all the args/progress handling removed and more thorough error handling.

Due to the dependency on rayon, the utility is currently gated behind the util feature of backhand.

I tried also gating the nix dependency on the feature, but it was hard due to it showing up in BackhandError.

Fixes: #354