Create a separate subclass for "ordered time" UUIDs

[ramsey]

UUIDs encoded and decoded as "ordered time" UUIDs often cause confusion, especially because their byte order doesn't appear to match their original byte order (see ramsey/uuid-doctrine#60, for example). This is a proposal to add a subclass for "ordered time" UUIDs, so that they can be separately identified and encoded/decoded without the confusion.

Perhaps this is related to #228

[ramsey]

A subclass for ordered-time UUIDs is no longer necessary, since version 4.0.0 now supports version 6 UUIDs, which are effectively the same as "ordered-time" UUIDs.