Fix dynamic SubAccess of zero-length vectors (bp #1450) (#1501)

* Fix dynamic SubAccess of zero-length vectors
* Fixes #230
* Add new ZeroLengthVecs pass that occurs before RemoveAccesses
* Include this in stage.Forms.MidForm
* Add to High->Mid order in compiler test based on @seldridge feedback
* Use validif to produce out-of-bounds value in ZeroLengthVecs
* Update scaladoc
* Fix test imports
* Delete DependencyAPI stuff for backport

(cherry picked from commit 1a03e635

)

Co-authored-by: Albert Magyar <albert.magyar@gmail.com>