Evolution: Make UUIDv1 public

[baarde]

UUID.UUIDv1, UUID.Timestamp, UUID.ClockSequence and UUID.NodeID are currently implementation details of the UUID.v1() method (I originally intended .v1() to just be a wrapper around uuid_generate_time() but I wasn't satisfied with the implementation in swift-corelibs-foundation).

Some developers might want to inspect time-based (version 1) UUIDs or generate UUIDs with custom node IDs (or timestamps), which is why I plan to make these types public in a future release.

However I first need to make sure that the APIs are good enough that I won't have to change them and break existing code in the near future. Also I have to write the corresponding docs.

Feel free to comment if you see anything that should change before I do.