Constants for standard UUIDs used as namespaces

We currently have a constant for the Null UUID, but there are others that are widely used as namespaces in V3 and V5 UUIDs:

> For example, some name spaces are the
   domain name system, URLs, ISO Object IDs (OIDs), X.500 Distinguished
   Names (DNs), and reserved words in a programming language.

List of constants: https://tools.ietf.org/html/rfc4122#appendix-C