WIP(do not merge): u256 to Felt procedure PR rebase visualization

[partylikeits1983]

Visualization of #2331 once rebased to agglayer-new

[mmagician]

I would prefer not to introduce another data type (U256), because the conversions are already pretty complex with the BE/LE conventions.

e.g. in:

#[tokio::test]
async fn test_scale_down_wrong_y_clean_case() -> anyhow::Result<()> {
    let x = U256::from_dec_str("10000000000000000000").unwrap();
    assert_y_plus_minus_one_behavior(x, 18).await
}

It would be much better if we stick to raw bytes [u8; 32], though for convenience we can wrap it in a struct like we do for ExitRoot, SmtNode etc. so that instead we have:

pub struct U256([u8; 32]);

impl U256 {
    fn from_uint_str(...) -> Self {...};
}

// then for conversion:
let x: = U256::from_uint_str("10000000000000000000)";

(the difference between using U256 is that it's a foreign struct which internally does not represent the values as raw bytes)

[mmagician]

I believe this can be closed since #2331 already contains the git history here