Declare string literal as frozen

Summary:
Ruby 3 will use frozen string litteral by default.

As Tommy strings are immutable, we can already switch to this behavior.

References:

• https://wyeworks.com/blog/2015/12/1/immutable-strings-in-ruby-2-dot-3
• https://bugs.ruby-lang.org/issues/11473
• https://bugs.ruby-lang.org/issues/8976#note-41

Test Plan: Run server

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1787