dist_array: add dist_array_id with here() and when_here()

[Dan Bonachea]

Copied from extras PR #12:

resolutions in our 2020-02-20 meeting

inclusion of a method to retrieve a TriviallySerializable dist_array_id with here() and when_here() methods to return the local representative.

This is not yet fully designed, but the implementation should probably be a second dist_object member that contains a pointer to the local representative, and we return a dist_array_id that encapsulates the dist_id from that collective object.

[Dan Bonachea]

Mass roll-over of open issues to next release milestone

[Dan Bonachea]

Mass roll-over of open issues to next release milestone

[Dan Bonachea]

dist_array lives in upcxx-extras and is not coupled to UPC++ release milestones