- Given a `KeyAmalgamation`, it is often necessary to determine the
    certificate's revocation status, or whether it is alive.

  - It is possible to get the `Cert` using `KeyAmalgamation::cert`,
    and to query it directly, but then the referece time needs to be
    provided.

  - To make it harder for the user to pass the wrong reference time,
    and to simplify the code, add methods to query the certificate
    using the KeyAmalgamation's reference time.