Aliases traverseVoid/sequenceVoid and their counterparts

[satorg]

Closes #4611.

The feedback on the original proposal seems generally favorable, therefore here is the PR. Overall, it does the following:

• adds traverseVoid/sequenceVoid to Foldable (convering traverse_ and sequence_ to aliases for the new methods);
• similarly, adds nonEmptyTraverseVoid/nonEmptySequenceVoid;
• similarly, adds corresponding new methods to Parallel/NonEmptyParallel;
• redirects customized implementations in data instances to the new methods;
• makes laws and tests using the new methods instead of the old ones.

Also it adds some comments to the places that haven't been changed but could be and thereby explaining why.

[satorg]

@ken1ma, fyi, because you referenced the original inssue in #4661 (comment)

[johnynek]

I think this is a good change. I left a comment about making a stronger recommendation than "consider".

Side note, really "foreach" is what this semantically is, but I think using that name is dangerous given that scala already uses that for side-effecting functions.

Maybe foreachA is also a potential name for this function which is more about the "what" whereas "traverseVoid" is kind of the how...

That said, I think traverseVoid/sequenceVoid are more consistent with cats style and a smaller change from the current situation and give us a clearer path to deprecate the foo_ naming style.

[johnynek]

I wouldn't say consider switching I would say replaced by traverseVoid.

[satorg]

@johnynek , thank you for the comment. Before I start making changes, I'd like to make sure we are on the same page. Are you suggesting to rephrase the notice this way (full sentence):

@deprecated this method should be considered as deprecated and replaced by `traverseVoid`.

Is it correct?

[johnynek]

yes. It is strange to me to deprecate one method, introduce an identical alias but then say "consider using the new version". I think it is more clear to say "please use the new version", or "calls to should be replaced to the new version" or something. Consider implies in my opinion that you may consider it and have a good reason not to, but if we thought there was a good reason not to, we shouldn't deprecate the current name.

[satorg]

Totally makes sense to me, thanks!

[johnynek]

Thanks for doing this!