Fixed a bug in the Order and PartialOrder instances for Tuple2+ where only the first element was used in comparisons

[mpilquist]

Before this fix:

scala> import cats.implicits._
import cats.implicits._

scala> (1 -> 2) === (1 -> 3)
res0: Boolean = true

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> showCode(reify { (1 -> 2) === (1 -> 3) }.tree)
res1: String = implicits.eqSyntax(Predef.ArrowAssoc(1).->(2))(implicits.tuple2Order(implicits.intOrder, implicits.intOrder)).===(Predef.ArrowAssoc(1).->(3))

This is because we pick up the Order[Tuple2[..]] instance, which is erroneously defined like this:

  implicit def tuple2Order[A0, A1](implicit A0: Order[A0], A1: Order[A1]): Order[(A0, A1)] =
    new Order[(A0, A1)] {
      def compare(x: (A0, A1), y: (A0, A1)): Int =
        A0.compare(x._1, y._1)
    }

I think we should probably publish a 0.6.1 with this fix too as it is really nasty -- code compiles but values end up being report equal which aren't.

/cc @non

[non]

👍 ugh thanks for the catch!

[non]

(we need more tests for the generated code!)

[ceedubs]

Yikes :(. Thanks!

[ceedubs]

It'd probably be nice to put this into a patch release. I won't have much availability to help with that for the next few days though :(