Override Function0.apply() with matching parens

[retronym]

Fixes #238

[retronym]

sbt:scala-async> console
[info] Starting scala interpreter...
Welcome to Scala 2.13.2 (OpenJDK 64-Bit Server VM, Java 1.8.0-adoptopenjdk).
Type in expressions for evaluation. Or try :help.

scala> :settings -Xlint:nullary-override

scala>

scala> import scala.concurrent._, ExecutionContext.Implicits.global, scala.async.Async._
import scala.concurrent._
import ExecutionContext.Implicits.global
import scala.async.Async._

scala> async { "hello" }
val res0: scala.concurrent.Future[String] = Future(<not completed>)