Extend issued-by? to support X509CRLHolder

When you deserialize a CRL as created by generate-crl, you don't get
the same CRL class back - you get a BouncyCastle X509CRLHolder.
This commit adds support for this class to the issued-by? multimethod.
I am very much in favor of moving all these to a test utils namespace, and keeping the production API minimal.