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.
well, I'm actually saying that I'd prefer them to be in other projects if the other projects actually need them.

I think we're nearing diminishing returns on this thread, though.  If we get rid of the 'CRLHolder' as @nwolfe and I were discussing, that will eliminate a good portion of my distaste for this, and if I'm the only one who has any distaste left after that change then I'm happy to just let @nwolfe make a call.