Add Show for Option and OptionT

The OptionT Show instance just uses the Show instance for the wrapped
`F[Option[A]]` as opposed to wrapping it in `OptionT(...)`. This is to
match the current behavior of `XorT`, but I'm wondering if that's
something we would want to change on both. It seems to me like the Show
output should reflect the fact that the value is wrapped in a monad
transformer, but I don't have particularly strong feelings about this.

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

@@ -88,6 +88,8 @@ final case class OptionT[F[_], A](value: F[Option[A]]) {
 
   def toLeft[R](right: => R)(implicit F: Functor[F]): XorT[F, A, R] =
     XorT(cata(Xor.Right(right), Xor.Left.apply))
+
+  def show(implicit F: Show[F[Option[A]]]): String = F.show(value)
 }
 
 object OptionT extends OptionTInstances {
@@ -138,4 +140,7 @@ trait OptionTInstances extends OptionTInstances1 {
     }
   implicit def optionTEq[F[_], A](implicit FA: Eq[F[Option[A]]]): Eq[OptionT[F, A]] =
     FA.on(_.value)
+
+  implicit def optionTShow[F[_], A](implicit F: Show[F[Option[A]]]): Show[OptionT[F, A]] =
+    functor.Contravariant[Show].contramap(F)(_.value)
 }

core/src/main/scala/cats/std/option.scala

@@ -79,6 +79,14 @@ trait OptionInstances extends OptionInstances1 {
             if (y.isDefined) -1 else 0
         }
     }
+
+  implicit def showOption[A](implicit A: Show[A]): Show[Option[A]] =
+    new Show[Option[A]] {
+      def show(fa: Option[A]): String = fa match {
+        case Some(a) => s"Some(${A.show(a)})"
+        case None => "None"
+      }
+    }
 }
 
 trait OptionInstances1 extends OptionInstances2 {

tests/src/test/scala/cats/tests/OptionTTests.scala

@@ -111,6 +111,11 @@ class OptionTTests extends CatsSuite {
     }
   }
 
+  test("show"){
+    val xor: String Xor Option[Int] = Xor.right(Some(1))
+    OptionT[Xor[String, ?], Int](xor).show should === ("Xor.Right(Some(1))")
+  }
+
   checkAll("OptionT[List, Int]", MonadCombineTests[OptionT[List, ?]].monad[Int, Int, Int])
   checkAll("MonadOptionT[List, ?]]", SerializableTests.serializable(Monad[OptionT[List, ?]]))
 

tests/src/test/scala/cats/tests/OptionTests.scala

@@ -12,4 +12,13 @@ class OptionTests extends CatsSuite {
 
   checkAll("Option[Int] with Option", TraverseTests[Option].traverse[Int, Int, Int, Int, Option, Option])
   checkAll("Traverse[Option]", SerializableTests.serializable(Traverse[Option]))
+
+  test("show") {
+    none[Int].show should === ("None")
+    1.some.show should === ("Some(1)")
+
+    forAll { fs: Option[String] =>
+      fs.show should === (fs.toString)
+    }
+  }
 }