Convert large params to small without copying out of Arc's

[DrPeterVanNostrand]

Allow direct conversion of MPCParameters into MPCSmall without copying large vectors. Something like the following (this almost certainly doesn't compile):

impl From<MPCParameters> for MPCSmall {
    fn from(large: MPCParameters) -> Self {
        let MPCParameters {
            params: Parameters {
                vk: VerifyingKey {
                    delta_g1,
                    delta_g2,
                    ..
                },
                h,
                l,
                ..
            },
            cs_hash,
            contributions,
         } = large;
        
        let h = h.try_unwrap().unwrap();
        let l = l.try_unwrap().unwrap();
        
        MPCSmall {
            delta_g1,
            delta_g2,
            h,
            l,
            cs_hash,
            contributions,
        }
    }
}