Change prefix stack to List[String] (bp #1617) (#1628)

* Change prefix stack to List[String] (#1617)

This allows for sharing of the prefix stacks between Data, and removes a
boxing per prefix stack element that is no longer necessary.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit fef0b68a

)

* Add binary compatibility waivers

Co-authored-by: Jack Koenig <koenig@sifive.com>