Function sapling_crypto::note_encryption::prf_ock

pub fn prf_ock(
    ovk: &OutgoingViewingKey,
    cv: &ValueCommitment,
    cmu_bytes: &[u8; 32],
    ephemeral_key: &EphemeralKeyBytes
) -> OutgoingCipherKey

Sapling PRF^ock.

Implemented per section 5.4.2 of the Zcash Protocol Specification.