Support v5 API URLs

Version 5 of the Sierra API did not introduce any changes to record ids, except the v4 part becomes v5.

New RelativeV5ApiUrl extends RelativeV4ApiUrl and new AbsoluteV5ApiUrl extends AbsoluteV4ApiUrl.

The existing record type can be converted to and from the new v5 API URL types.

New class-based API

The API has been completely re-written to be a hierarchy of classes.

The API should be more stable now. I don't expect any more radical re-writes. 😀

convertToAsync is still not implemented. So we still cannot convert to or from database ids for virtual records. 😞

Initial release

Functional, except convertAsync is not implemented.
API is still experimental and is subject to (possibly radical) change.