Implement versioning scheme for all crdts

[lenary]

This means the extra functions that I added to pncounter when I did my v2 work:

• to_binary/2
• from_binary/2
• current_version/1
• change_versions/3

I think we should be ready for upgrades, and implement these functions all now, so we only have to add to them later.

[seancribbs]

I think this can be addressed in a later release. Assigning 2.1