Tweak EitherT.pure and rightT to avoid a map call (#3408)

It's not necessary.

core/src/main/scala/cats/data/EitherT.scala

@@ -689,7 +689,7 @@ object EitherT extends EitherTInstances {
    * Uses the [[http://typelevel.org/cats/guidelines.html#partially-applied-type-params Partially Applied Type Params technique]] for ergonomics.
    */
   final private[data] class PurePartiallyApplied[F[_], A](private val dummy: Boolean = true) extends AnyVal {
-    def apply[B](b: B)(implicit F: Applicative[F]): EitherT[F, A, B] = right(F.pure(b))
+    def apply[B](b: B)(implicit F: Applicative[F]): EitherT[F, A, B] = EitherT(F.pure(Right(b)))
   }
 
   /**