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)