128-bit version for uuidv7

[saolof]

Postgres 18 was supposed to have direct support for uuidv7, and even if it does not gain it uuidv7 is somewhat common as application generated uuids. The downside of uuidv7 vs uuidv4 is predictable generation, Feistel cyphers remove that problem. It'd be nice to have a way to obfuscate a uuidv7 into a uuidv4