Mixin: fix the initialization of traits

[smarter]

Before this commit, the following code:

trait Hello {
  println("Hello")
  val x: Int = 1
  println("World")
}

Became:

<trait> trait Hello extends Object {
  def <init>(): Hello = {
    {
      ()
    }
    this
  }
  <accessor> def x(): Int
  protected def initial$x(): Int = {
    println("Hello")
    1
  }
}

Notice that the initialization statements after the last getter were
missing, this is now fixed:

<trait> trait Hello extends Object {
  def <init>(): Hello = {
    {
      println("World");
      ()
    }
    this
  }
  <accessor> def x(): Int
  protected def initial$x(): Int = {
    println("Hello")
    1
  }
}

Review by @odersky

[scala-jenkins]

Can one of the admins verify this patch?

[smarter]

Rebased.