Function commit_data 

pub fn commit_data<E: Curve>(
    rng: &mut (impl RngCore + CryptoRng),
    data: Data<E>,
) -> (Commitment<E>, PrivateCommitment<E>)

First round of the protocol: commit to well-constructed Data by itself. Produces Commitment to be sent

rng is used to generate the nonce for the private commitment