[vector] Stop leakage of internal APIs

[Paul Sandoz]

Hi,

This patch stops the leakage of package private Abstract* classes into the public API:

  http://cr.openjdk.java.net/~psandoz/panama/vector-internal-api-leakage/webrev/ <http://cr.openjdk.java.net/~psandoz/panama/vector-internal-api-leakage/webrev/>

This applies to: 

1) public methods overridden by the abstract classes that are not themselves fully overridden by further public subclasses; and
2) those methods return a type of that is a package private abstract class.

This primarily affects Vector. AbstractVector and public subclasses, such as DoubleVector.  I also made changes to species, shuffle, and mask classes for consistency even though there are no public subclasses.

Paul.