[MLton] power pc "port"

[Matthew Fluet]

> 1) Separate all word functions into signed and unsigned.

We used to do this, and I suspect that this is the right solution.

> 3) Implement sub-32-bit words by embedding them in a 32-bit word at the
> SML level.  Will this violate any specifications?

I don't think it violates anything -- it's what we do with the
non-standard integer sizes.  It's just a performance hit on those
architectures that support operations at the smaller sizes.