SI-7373 Make the constructor of Vector non-public

The danger of:

   new Vector(1, 2, 3).toString
   java.lang.NullPointerException

and the "should have been private all along" argument
call for a break in the source compatibility policy here.

src/library/scala/collection/immutable/Vector.scala

@@ -59,7 +59,7 @@ object Vector extends IndexedSeqFactory[Vector] {
  *  @define mayNotTerminateInf
  *  @define willNotTerminateInf
  */
-final class Vector[+A](private[collection] val startIndex: Int, private[collection] val endIndex: Int, focus: Int)
+final class Vector[+A] private[immutable] (private[collection] val startIndex: Int, private[collection] val endIndex: Int, focus: Int)
 extends AbstractSeq[A]
    with IndexedSeq[A]
    with GenericTraversableTemplate[A, Vector]