SSA backend

[Matthew Fluet]

> > Fine for now.  I'm going to run the usual battery of tests, and if everything
> > passes, I'd like to merge the SSA branch into the main branch and have everyone
> > continue from there.
> 
> Whoops.  It looks like you didn't check in your change to
> remove-unused.new.fun.  How about I run the battery of tests with
> remove-unused.fun, and if that all passes, move everything over?

Sorry, longer lunch than ususal.  If you haven't started running them,
I'll check in the new version of remove-unused.new.fun.  Otherwise, I'll
hold off until after the merge and copy it over to the main branch.