Move UUID class from Strings to Identifiers namespace

Summary:
This is a follow-up for 8eccc724dbe9.

• "Strings" is a language, lower level concept
• "Identifiers" is a functional, higher level concept

Test Plan: Run tests

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1611