Repercussion of latest Num addition changes

[tchataigner]

Issue

The recent PR #88 introduces a new behavior for Num::add, favouring None to Some(v) as a result in (Some(v), None) | (None, Some(v) cases.

This new behavior seems to break the current implementation of PoseidonROCircuit::squeeze in arecibo. In this method we actually call the SpongeCircuit::absorb method with the current state elements, that do not have any values at that stage.

In neptune absorb will call add_rate_element with a Num::add that results on a None variant, transmitted to the set_element method that leverages an unwrap on the value and thus panicking.

I'm not sure about the correct way to fix this, so I'm looking for inputs.

How to reproduce

This bug has been found while leveraging the Bn256EngineKZG engine at PublicParams::setup both for Nova & Supernova. You can reproduce it on your machine by running this test on ethereum-lc/aggpk

I was also able to reproduce it on the aptos-lc if patching both belpepper & bellpepper-core to dev.