Add new Rails/IndexBy and Rails/IndexWith cops

[eugeneius]

Active Support provides Enumerable#index_by, which creates a hash by yielding each element and using the result as the key and the original element as the value, and Enumerable#index_with, which uses the result as the value and the original element as the key.

These cops identify and correct some patterns for doing the same thing using each_with_object, map { ... }.to_h, or Hash[map { ... }].

This is heavily based on @djudd-stripe's work in rubocop/rubocop#7663, adapted to detect the same patterns when called on an enumerable rather than a hash.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

Great 👍