IM instances need to be "forwarded" in Cartesian companion

This example (which is part of the tut):

```scala
import cats.std.all._
import cats.syntax.all._

// Let's build a Semigroup for this case class
case class Foo(a: String, c: List[Double])

implicit val fooSemigroup: Semigroup[Foo] = (
  (implicitly[Semigroup[String]] |@| implicitly[Semigroup[List[Double]]])
    .imap(Foo.apply)(Function.unlift(Foo.unapply))
)
```

Fails with

```
<console>:23: error: value |@| is not a member of cats.Semigroup[String]
         (implicitly[Semigroup[String]] |@| implicitly[Semigroup[List[Double]]])
```

If I remove the following explicit "forwarding" of `InvariantMonoidal` instances in `Cartesian` companion object:

```
object Cartesian extends CartesianArityFunctions with AlgebraCartesianInstances

/**
 * Cartesian instances for types that are housed in Algebra and therefore
 * can't have instances for Cats type classes in their companion objects.
 */
private[cats] sealed trait AlgebraCartesianInstances {
  implicit val invariantSemigroup: Cartesian[Semigroup] = InvariantMonoidal.invariantMonoidalSemigroup
  implicit val invariantMonoid: Cartesian[Monoid] = InvariantMonoidal.invariantMonoidalMonoid
}
```

Here is the output from travis for reference:

- https://travis-ci.org/typelevel/cats/jobs/138068094

core/src/main/scala/cats/Cartesian.scala

@@ -16,13 +16,4 @@ import simulacrum.typeclass
   def product[A, B](fa: F[A], fb: F[B]): F[(A, B)]
 }
 
-object Cartesian extends CartesianArityFunctions with AlgebraCartesianInstances
-
-/**
- * Cartesian instances for types that are housed in Algebra and therefore
- * can't have instances for Cats type classes in their companion objects.
- */
-private[cats] sealed trait AlgebraCartesianInstances {
-  implicit val catsInvariantSemigroup: Cartesian[Semigroup] = InvariantMonoidal.catsInvariantMonoidalSemigroup
-  implicit val catsInvariantMonoid: Cartesian[Monoid] = InvariantMonoidal.catsInvariantMonoidalMonoid
-}
+object Cartesian extends CartesianArityFunctions