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.
The multimethod still doesn't bother me that much but maybe that's because I'd been thinking of them more as testutils than as "production" code.  I'd be okay with just having the caller deal with the actual types if it didn't lead to a lot of repetition but it does seem like these provide some utility across a group of tests.  I'd support moving them into a testutils namespace to be more formal how we are treating these.