Method for checking if one certificate issued another

This lets you check if one certificate was issued by another one.

    > print(parent:issued(child))
    true

The backing OpenSSL function is undocumented and I believe that it performs only issuer name matching and signature verification.  E.g. Basic Constraints seem to be ignored.

I needed this for doing DANE-TA (usage 2) as part of my DANE implementation for Prosody.

Thoughts?  I'm not sure this is ready for merging, so I would like some feedback on if this is something useful.