Rename gem and make it Rails-agnostic

[ivy]

Thoughts on renaming this gem to ruby_at_scale-redis and making it Rails-agnostic? This looks small enough that you might only have to:

• Rename the gem
• Create a deprecated constant for BigRails::Redis
• Swap the ActiveSupport::Autoload dependency with Zeitwerk

[dpep]

I like the idea of making this gem rails optional. If Rails is present, sprinkle in the Rails.application.redis.for helpers. Otherwise, the functionality can be exposed elsewhere. The change might also require us to audit the library to ensure we're not using any rails helpers, and upgrading the specs to run with/without rails installed