RSSA backend

[Matthew Fluet]

> Great.  Other than that, what's currently checked in to the rssa branch
> passes all the regressions, benchmarks, and a self-compile.  

If you decide to merge RSSA branch in, I've got no objections.  Playing
around with the Random.alphaNumString "bug" earlier, I only say conflicts
with ssa-tree.fun, which are all easy to fix.  You'll probably just want
to replace src/mlton/backend completely, rather than trying to deal with
the conflicts.  (Just got you're cvs experiment messages, so maybe there
aren't as many as I would have expected.)  Just one other note, do merge
in the x86-codegen directory, because there are a few revisions on the
main branch that aren't in the RSSA branch.