Add unorderedFoldMapA method

[m50d]

Slightly trivial, but it's a method I'd expect to be there, a natural counterpart of foldMapA.

[johnynek]

seems worth having to me. That said, I think CommutativeApplicative may be underused, so we there could be instances we don't label.

[satorg]

@m50d , thank you for the PR!

Would you be open to add a rule to UnorderedFoldableLaws? Since unorderedFoldMapA can be implemented in terms of unorderedFoldMap, then the rule could look like "unorderedFoldMapA is consistent with unorderedFoldMap", or something like that.

[m50d]

@satorg I've added an identity law, it feels a bit overkill to add a full consistency law given that we don't have one for foldMapA (or maybe I should be adding them for both?)

[satorg]

Thank you @m50d!
You may also want to add this law to the corresponding rule set in laws/src/main/scala/cats/laws/discipline/UnorderedFoldableTests.scala.
Without that the law won't get checked.

[m50d]

Ah indeed - thank you!

[satorg]

Thank you, @m50d !