New impala lineage; develop from here

The original impala branch (now `impala-orig`) was quite diverged from
master/devel, and merging/rebasing was complex because of complex lineage.
Since the impala changes are primarily orthogonal to master/devel, I have
simply moved the new files into a new branch off of devel.  I will rebase
more frequently off of devel to make sure the two don't get too out of sync.