Add reordering to convert between v1 and v6 UUIDs

[tgross35]

Add reordering to convert between v1 and v6 UUIDs

[tgross35]

The authors of the v6 spec actually provide some conversion algorithms here: http://gh.peabody.io/uuidv6/ under the "hacks" section

[KodrAus]

Hmm, I'd rather not add methods to Uuid itself that effectively become quiet no-ops depending on the version. In this case, I think we could add a method like fn node_id(&self) -> Option<[u8; 6]> that returns Some for v1 and v6 UUIDs, so that you could then write:

if let (Some(ts), Some(node_id)) = (uuid.timestamp(), uuid.node_id()) {
    Uuid::new_v6(ts, &node_id)
} else {
    uuid
};

to get yourself a sortable UUID. It's a bit verbose, but keeps the scope of the library to constructing and deconstructing UUIDs, and not straying into grab-bag utility territory. That kind of functionality is useful, but it's difficult to gauge what's worth including.