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.
Yeah, I'm back and forth on this stuff too.  These certainly make it easier to write tests, but it's not necessarily clear where we draw the line with this stuff.

My experience with working with this library (jvm-ca) is that I have to frequently reference some javadocs in order to figure out how to do what, and after finding myself needing something multiple times I tend to prefer just creating a function for it (this PR).

But at the same time, I don't know how much I like having an "open-ended API" where we just add things as they are encountered.  Part of me thinks that there's no way we're ever going to know the entire set of classes that we should be supporting by these `issued-by?` predicates so do we really want to go down the path of abstracting over them? 

The contract of `issued-by?` is supposed to be that it abstracts the method of retrieving and testing against the "issuer" field on any SSL/CA-related objects.

So, I too am on the fence with this stuff, and I think we could also just make the case for having them be project-specific test utility functions that just live inside the test namespaces (or a shared test utils) that need them?  But then again, you could consider the jvm-ca to be the shared test utils.  What if this stuff wasn't part of the API of this library but just provided as a convenience for test utils?  Or is that still considered the API?