Function bls_on_arkworks::sign

pub fn sign(
    sk: SecretKey,
    message: &Octets,
    dst: &Octets
) -> Result<Signature, BLSError>

(spec link) Computes a signature from SK, a secret key, and message, an octet string.

The implementation is described as:

   1. Q = hash_to_point(message)
   2. R = SK * Q
   3. signature = point_to_signature(R)
   4. return signature

XXX: this function doesn’t take DST as an argument in the spec. It should!