RSSA backend

[Matthew Fluet]

> Merge of rssa branch.
> Contents of repository just before merge were tagged pre-rssa-merge.

CRAP!  It's been my experience that if regressions can get through array
and callcc, then we're set, but:

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