Convert ipaddresses to 4x API namespaced function

The modern API ruby functions don't have any environment isolation
issues, can be namespaced, perform automatic type checking etc.

Fixes #270

See
http://puppet-on-the-edge.blogspot.com/2015/01/the-puppet-4x-function-api.html
and
https://github.com/puppetlabs/puppet-specifications/blob/0286d30dc903f36c36879cef6f09a9226c8096fc/language/func-api.md