add sizeIs method like std-lib to cats.Reducible and/or NonEmpty collections

[xuwei-k]

https://github.com/scala/scala/blob/8ad582dec8ec6d671a36436766bc6c789d2cdbda/src/library/scala/collection/Iterable.scala#L290-L304

[satorg]

Perhaps, it would make sense indeed, but why do you think it should be limited to cats.Reducible and non-empty collections only? Currently, the Reducible's base trait UnorderedTraverse already has just size. Perhaps, it would be better to think about adding sizeIs/sizeCompare to the latter, wdyt?