complete Ipaddr interface
simply types
Trivial clean-up of @hhugo's interface completion
Make v4 a true embedding in v6, expose v4/v6 prefix convertors, expose generic prefix serializers
Prep for 2.1.0
Merge pull request #21 from mirage/v6

Raw prefix reading, address -> prefix injection, new version sum type, generic prefix module. Closes #17.