x509-cert: add a CRL builder

[baloo]

This adds a builder interface for CRLs.

This would accept an iterator for the certificates to be revoked.

    let builder = CrlBuilder::new(&ca_certificate, crl_number)?
        .with_certificates(
            vec![
                RevokedCert {
                    serial_number: serial_number1,
                    revocation_date: Time::now()?,
                    crl_entry_extensions: None,
                },
                RevokedCert {
                    serial_number: serial_number2,
                    revocation_date: Time::now()?,
                    crl_entry_extensions: None,
                },
            ]
            .into_iter(),
        );

    let crl = builder.build(&signer)?;