+slick #1329 add flatten to DBIOAction

slick-testkit/src/main/scala/com/typesafe/slick/testkit/tests/ActionTest.scala

@@ -109,4 +109,21 @@ class ActionTest extends AsyncTest[RelationalTestDB] {
      a5.map(_ shouldBe "a5")
    )
  } else DBIO.successful(())

  def testFlatten = {
    class T(tag: Tag) extends Table[Int](tag, u"t") {
      def a = column[Int]("a")
      def * = a
    }
    val ts = TableQuery[T]
    for {
      _ <- db.run {
        ts.schema.create >>
          (ts ++= Seq(2, 3, 1, 5, 4))
      }
      needFlatten = for (_ <- ts.result) yield ts.result
      result <- db.run(needFlatten.flatten)
      _ = result shouldBe Seq(2, 3, 1, 5, 4)
    } yield ()
  }
}

slick/src/main/scala/slick/dbio/DBIOAction.scala

@@ -49,6 +49,11 @@ sealed trait DBIOAction[+R, +S <: NoStream, -E <: Effect] extends Dumpable {
  def flatMap[R2, S2 <: NoStream, E2 <: Effect](f: R => DBIOAction[R2, S2, E2])(implicit executor: ExecutionContext): DBIOAction[R2, S2, E with E2] =
    FlatMapAction[R2, S2, R, E with E2](this, f, executor)

  /** Creates a new DBIOAction with one level of nesting flattened, this method is equivalent
    * to `flatMap(identity)`.
    */
  def flatten[R2, S2 <: NoStream, E2 <: Effect](implicit ev : R <:< DBIOAction[R2,S2,E2]) = flatMap(ev)(DBIO.sameThreadExecutionContext)

  /** Run another action after this action, if it completed successfully, and return the result
    * of the second action. If either of the two actions fails, the resulting action also fails. */
  def andThen[R2, S2 <: NoStream, E2 <: Effect](a: DBIOAction[R2, S2, E2]): DBIOAction[R2, S2, E with E2] = a match {