Implement Ciphertext::from_elements

[dobo90]

What?

This is similar to PR34. It implements additional function in Ciphertext. I've named it from_elements as there are already from_element functions in PublicKey and VerifiableDecryption (of course I can change to from_parts or any other name). For example it allows to instantiate Ciphertext from untagged bytes:

use elastic_elgamal::{group::Generic, Ciphertext};
use p256::{elliptic_curve::sec1::FromEncodedPoint, AffinePoint, EncodedPoint, NistP256};

fn main() {
    let random_element_bytes = [0u8; 64]; // some real bytes here
    let blinded_element_bytes = [0u8; 64]; // some real bytes here

    let random_element = EncodedPoint::from_untagged_bytes(&random_element_bytes.into());
    let random_element = AffinePoint::from_encoded_point(&random_element).unwrap();

    let blinded_element = EncodedPoint::from_untagged_bytes(&blinded_element_bytes.into());
    let blinded_element = AffinePoint::from_encoded_point(&blinded_element).unwrap();

    let ciphertext: Ciphertext<Generic<NistP256>> =
        Ciphertext::from_elements(random_element.into(), blinded_element.into());
}

Why?

playready-rs requires a functionality to instantiate Ciphertext from bytes sent over the wire. For now I've forked elastic-elgamal but I would love to publish playready-rs on crates.io (but it's not possible as it uses modified version of this repository).