Re-design hades permutation

[moCello]

Summary

Expose only permute and permute_gadget functions (and WIDTH) from hades module.
Edit: After a comment from @xevisalle we decided to only expose the WIDTH from the hades module to the public API.

With this change we can refactor the permutation designs internally without a breaking change, if desired, in the future.

[moCello]

Following up the comment from #242 (comment)
@xevisalle do you think the entire permutation functions should better be private?

[xevisalle]

Following up the comment from #242 (comment) @xevisalle do you think the entire permutation functions should better be private?

will someone ever want to use hades without poseidon? as it is optimized for our implementation, and now it is even part of the poseidon crate, I guess it's unlikely to happen. In the future we can set public functions to change Hades configurations, but by now I would keep it private.

[moCello]

I agree. I'll only expose the WIDTH then.