Reference implementaiton for UUID v8

[yepher]

It would be nice to have a reference implementation for UUID v8 as defined here:

https://www.ietf.org/id/draft-peabody-dispatch-new-uuid-format-03.html

[gofish]

See https://github.com/uuid6/uuid6go-proto

[Dentrax]

See uuid6/uuid6go-proto

README mentions v8 but I don't think it generates real UUID v8 since I couldn't see a UUIDv8Generator implementation. cc @nurked