Truly editorial fix for comments. This works better with Emacs' set-justification-full function. [ci skip]
They are no longer how Object#clone/Object#dup are defined. In fact DUPSETUP is not used from anywhere. CLONESETUP has only one usage. Let's not expose them to extension libraries. cf https://github.com/ruby/ruby/pull/4100#discussion_r563481718
To fix build failures.
Devs do not love "impl".