[WIP] Migrate Arel code from Rails to Oracle enhanced adapter

[yahonda]

This is a draft pull request to support rails/rails#38946

Required

• Migrate Arel library code to make CI green
• Migrate git commit history

Optional

• Migrate test code
  because the unit test for Arel is written in Minitest, Oracle enhanced adapter uses RSpec. Arel has been a private API at Rails then it may be fine without tests for the time being.

[yahonda]

@koic I'd like to get your help how to migrate Arel library code with git commit history. I think you have done a similar job at rubocop/rubocop#5977 (comment)

[koic]

Yeah, I will try this one.

[koic]

I created the following branch to solve this.
https://github.com/koic/oracle-enhanced/tree/merge_arel_visitors

It passes specs using remove_unused_visitors branch of kamipo/rails repository.

gem "activerecord",   github: "kamipo/rails", branch: "remove_unused_visitors"

You can cherry-pick these commits into this PR or I can open a new PR If you'd like.
koic/oracle-enhanced@e7d3240...af14bb0

[yahonda]

@koic Thanks for the great work! Would you open a new pull request?

[koic]

I opened PR #2002. Thank you.

[yahonda]

Closing this draft pull request.