[Reviewer: Rob] Add support for protocol independent (IPv4/IPv6) table indices based upon RFC 4001.

I believe this is a way to move forward from the IpAddress table indices used in the current MIB. I'm not sure it is the best way. It is more of an experiment, and a place to start the discussion of how to proceed.
Agreed, that code is too terse and relies upon the pull request comment to make sense. I’ve added an empty block with an appropriate comment.