DRAFT: Transient `vec`s

[gefjon]

[gefjon]

Tabling this for now, because while implementing the various vec operators on transients I've discovered that transient vecs aren't a huge win - push-back and pop-back get better, but extend and retract already optimize batched insertions and removals to and from the end; replace-at and update-at can potentially re-use the body structure; but most operations will have to copy any changes to the body anyway because nodes (i.e. simple-vectors) aren't adjustable.