Implement generic Struct#clone

[Sija]

Followup to PR #4174 adding Struct#clone (internally calling #dup).

[bcardiff]

You don't wait :-)
clone should perform a deep copy

[Sija]

Sorry, too much ☕️ today turned my fast&furious mode ;) I was considering using def_clone, would that be sensible?

[Sija]

CI is failing because of a bug?

[asterite]

Just in case, read this

It was previously decided that no generic clone method will be provided. That it was missing in Time (and probably in other types in the standard library) is not a good reason to provide a generic implementation for this. Cloning every member isn't always what you want, so forcing an implementation when needed is better.

[Sija]

@asterite I was thinking the same, Struct#clone sounded like a big generalization, that's why I started with #4174.

[bcardiff]

Ok, my bad for suggesting a generic clone implementation for structs. Let's stay just with #4174.
I forgot about that conversation.