Use AnyVal to avoid allocations

Co-authored-by: Arman Bilge <armanbilge@gmail.com>

Author: lenguyenthanh

core/src/main/scala/cats/mtl/Handle.scala

@@ -230,8 +230,8 @@ object Handle extends HandleInstances with HandleVariant {
     def apply[A](body: Handle[F, E] => F[A])(implicit F: ApplicativeThrow[F]): Inner[A] =
       new Inner(body)
 
-    final class Inner[A](body: Handle[F, E] => F[A])(implicit F: ApplicativeThrow[F]) {
-      def rescue(h: E => F[A]): F[A] = {
+    final class Inner[A](private val body: Handle[F, E] => F[A]) extends AnyVal {
+      def rescue(h: E => F[A])(implicit F: ApplicativeThrow[F]): F[A] = {
         val Marker = new AnyRef
 
         def inner[B](fb: F[B])(f: E => F[B]): F[B] =