Merge pull request #1043 from ericsnowcurrently/backups-archive-workspace

Add a backup archive workspace type.
Add an ArchiveData wrapper type.
Expose the Open method.
Fix up some doc comments.
Drop a superfluous assignment.
NewArchiveFile -> NewArchiveData (and add data parameter).
af -> ad.
Pull ArchiveData out of the archive package.
Add a test for legacy version.
Adjust to upstream changes.