Add UUID module

[stilist]

I recently used FFaker::Guid.guid and realized it can produce invalid UUIDs: UUIDs can’t be arbitrary data; bits 48–51 and 64–65 must have predefined values. Rather than change Guid.guid to produce a valid UUID, I’ve deprecated it and replaced it with more-specific UUID.uuidv4, .uuidv6, .uuidv7, and .uuidv8 methods. These follow the specification (which is still being worked on, but close to approval — current draft).

• I chose not to implement UUID v1, v2, v3, or v5 because I don’t remember seeing them used in the past 15 years. They can be added to the module if needed, though.
• I debated whether to have the v6 and v7 methods use real timestamps by accepting from, to arguments like Date.between. I decided it wasn’t necessary, though it might be useful for testing code that relies on those versions’ behavior of putting the slowest-changing part of the timestamp at the start of the UUID. (For example, a trie.)