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

* 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