Function create_legacy_rsa_id_cert 

pub fn create_legacy_rsa_id_cert<Rng: CryptoRng>(
    rng: &mut Rng,
    now: SystemTime,
    hostname: &str,
    keypair: &KeyPair,
) -> Result<Vec<u8>, X509CertError>

Create an X.509 certificate, for use in a CERTS cell, self-certifying the provided RSA identity key.

The resulting certificate will be encoded in DER. Its cert_type field should be 02 when it is sent in a CERTS cell.

The resulting certificate is quite minimal, and has no unnecessary extensions.

Returns an error on failure, or if keypair is not a 1024-bit RSA key with exponent of 65537.