CoreVerifier setup does not compute total_stake correctly

[jpraynaud]

Why

The Coreverifer of the mithril-stm does not correctly compute the total_stake during the setup: this can lead to division by zero when the num-integer-backend is used.

What

The total_stake variable is not mutable and is always 0 when returned by the setup function:

pub fn setup(public_signers: &[(VerificationKey, Stake)]) -> Self {
    let total_stake: Stake = 0;
    let mut unique_parties = HashSet::new();
    for signer in public_signers.iter() {
        let (_, overflow) = total_stake.overflowing_add(signer.1);
        if overflow {
            panic!("Total stake overflow");
        }
        unique_parties.insert(MTLeaf(signer.0, signer.1));
    }

    let mut eligible_parties: Vec<_> = unique_parties.into_iter().collect();
    eligible_parties.sort_unstable();
    CoreVerifier {
        eligible_parties,
        total_stake,
    }
}

The rug backend does not detect a division by zero, probably because it is converted to a float before dividing.

How

• Fix the computation of the total_stake when creating the CoreVerifier
• Add a test that verifies the computation of total_stake