Deduplicate Active Record reflection names

[casperisfine]

While reading some memory profiles I noticed some common string duplication in Reflection:

Retained String Report
-----------------------------------
423  "shop"
361  /tmp/bundle/ruby/2.5.0/bundler/gems/rails-9b5401fcc962/activerecord/lib/active_record/reflection.rb:25

Unsurprisingly that 361 maps to the number of belongs_to :shop we have. So since that string is effectively used as a symbol, I think it makes sense to freeze and deduplicate it, also since it's derived from model names, it's likely to be used across Rails and the application codebase.

This won't be a huge saving, even for bigger apps, but it's also a very simple one to implement so it might be worth it.

@rafaelfranca @Edouard-chin