Reference encryption key in ciphertexts

[mortendahl]

By also having a reference to the corresponding encryption key in ciphertexts we can making homomorphic operations easier to read, ie c1 + c2 and c * 5 instead of Paillier::add(ek, c1, c2) and Paillier:mul(c, 5).

Implementation wise this could be done by simply redirecting to the existing Paillier method and implementing Add and Mul for ciphertexts.