ArrowAssoc replaced with an extension method

scala2-3-ArrowAssocExtensionMethod.scala

// From https://github.com/deanwampler/programming-scala-book-code-examples/
import scala.annotation.targetName

extension [A,B] (a: A)
  @targetName("arrow2") def ~>(b: B): (A, B) = (a, b) 

// Alternative, where the [B] goes on the method instead. Support for this alternative was
// introduced in Scala 3.0.0-RC1. This form is closer to what we're used to doing with
// normal parameterized types and methods that take additional type parameters. the type 
// signature for the two ~> methods will be slightly different, but they work the same.

extension [A] (a: A)
  @targetName("arrow2") def ~>[B](b: B): (A, B) = (a, b)