Use R6 for more consistent behavior with Schema Docs

[collinschwantes]

Is your feature request related to a problem? Please describe.
This package mixes object oriented and function methods in a way that may confuse users.
Right now we rely on the to store things that would fit better in an r6 object.

Describe the solution you'd like
Use R6 classes to create objects which contain their own environment

With R6 objects, you could compare versions more easily.