RSSA backend

[Matthew Fluet]

> testing fft
> Type error: analyze raised exception x_1: loopTransfer: HandleOnly
> impossible_0 (global_0,x_0): Vector.fold2
> 
> mlton: unhandled exception: TypeError
> compilation of fft failed
> testing filesys
> Type error: analyze raised exception fileId_0: loopTransfer: L_0 (concat_0
> (x_1,x_0)): Vector.fold2
> 
> mlton: unhandled exception: TypeError
> compilation of filesys failed

Why am I plagued with inexplicable regression errors?  I can't reproduce
these either locally or on the machine where the rest of the regressions
are still going (i.e., over on that machine, I'm using exactly the same
executable and regression files that yielded the above).

Alright, it only seems to crop up when running in the regression script.