Don't allow redefinition of core classes

[odersky]

Fixes #1688. Review by @felixmulder

[felixmulder]

This gets through compilation and prints 5:

object scala {
  trait Boolean {
    def x: Int
  }
}

object Test {
  def x: scala.Boolean = new scala.Boolean { val x = 5 }

  def main(args: Array[String]): Unit = println(x.x)
}

Not sure if the first object lands in <root> or <empty>

[felixmulder]

Discussed in person - this is fine because reasons (where reasons == bootstrap)

[felixmulder]

LGTM :)