Add new Style/RedundantEach cop

[koic]

Follow up https://github.com/rubocop/rubocop-rails/pull/812/files#r994033301.

This PR adds new Style/RedundantEach cop that checks for redundant each.

This cop is unsafe, as it can produce false positives if the receiver is not an Enumerable.

# bad
array.each.each { |v| do_something(v) }

# good
array.each { |v| do_something(v) }

# bad
array.each.each_with_index { |v, i| do_something(v, i) }

# good
array.each.with_index { |v, i| do_something(v, i) }

# bad
array.each.each_with_object { |v, o| do_something(v, o) }

# good
array.each.with_object { |v, o| do_something(v, o) }

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

I like the suggestion, but I'm wondering if this is not more of a Lint cop.

[bbatsov]

I'm guessing that using each_with_index directly is also good. Same with each_with_object. Probably we'll need some separate cop about such iterators (e.g. something suggesting each.with_object over each_ with_object).

[koic]

Sure,each_with_index is also accepted. I've updated it!
OTOH, I did a quick check and saw no difference in performance. This autocorrection chooses each.with_object because the receiver is an Enumerator and using method chain. But, I'm not sure which of each_with_object and each.with_object is definitely better as the default when receiver is a collection object (e.g. instance of Array, Hash, Set).

[koic]

I like the suggestion, but I'm wondering if this is not more of a Lint cop.

I was a little worried, but it's unsafe, so I chose Style over Lint. It's also because the original code is not a dangerous problem. For example, reverse_each.each_with_index would be a favorite style to eliminate the redundant expression each rather than a definite mistake.

[bbatsov]

Thanks!