RSSA backend

[Stephen Weeks]

> Hint: bash programs are _not_ lexically scoped.

Ah.  I checked in a fix.

> Looks pretty good.  A couple of slowdowns, but nothing compared to some of
> the speedups.

I'll say.  Some surprisingly good speedups.  It's funny how when you
write an optimization, you often get no speedup, but when you do some
restructuring of the compiler, you get some really good ones.

Unfortunately, it looks like barnes-hut and nucleic will continue to
prevent us from being faster than every other SML compiler on every
benchmark.