Cats: Tuple Apply syntax fixes

[adelbertc]

Referring to this syntax:

import cats.implicits._

(Option(2), Option(3)).map2(_ + _)

• [ ] No Unapply
• [ ] Should this be under cats.syntax.tuple as it currently is or cats.syntax.apply?
• [ ] Deprecate/remove CartesianBuilder syntax

[non]

For (1), let's add it!
For (2) I think I'd prefer cats.syntax.apply but I don't feel super strongly if someone else disagrees.
For (3), yes.

[longcao]

As noted by @non, mapN would be a nice name for this: https://gitter.im/typelevel/cats?at=582dd60269eb65323c45819c

[mpilquist]

:+1: for mapN so that folks don't need to manually count the number of values.

[julienrf]

for Unapply support I remember having tried without success a few month ago. You can see my code here.

[mpilquist]

@julienrf I think we can drop Unapply support now that we have SI-2712 fixed.