Function verify_signatures 

pub(crate) fn verify_signatures<T, D, V>(
    cert: &SimpleCertificate<T, D, V>,
    stake_table: &[<T::SignatureKey as SignatureKey>::StakeTableEntry],
    threshold: U256,
    upgrade_lock: &UpgradeLock<T>,
) -> Result<()>
where
    T: NodeType,
    D: Voteable<T> + 'static,
    V: Threshold<T>,
    SimpleCertificate<T, D, V>: Certificate<T, D>,

Verify a certificate’s signatures, at the genesis view as at any other.

TODO: Almost a duplicate of SimpleCertificate::is_valid_cert.