Use AnyVal to avoid allocations

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

Author: lenguyenthanh

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

@@ -225,7 +225,7 @@ object Handle extends HandleInstances with HandleVariant {
   def allowF[F[_], E]: AdHocSyntaxTired[F, E] =
     new AdHocSyntaxTired[F, E]
 
-  final class AdHocSyntaxTired[F[_], E] {
+  final class AdHocSyntaxTired[F[_], E](private val unit: Unit) extends AnyVal {
 
     def apply[A](body: Handle[F, E] => F[A])(implicit F: ApplicativeThrow[F]): Inner[A] =
       new Inner(body)