[MLton] speeding up Real.abs

[Stephen Weeks]

> It would be nice if this new structure call it "NativeReal" or 
> "FastReal" had a compatible same signature as "Real".
> 
> Then if I really don't care about ieee I can just say
> structure Real =  structure FastReal;
> 
> and be done with it for 90% of the cases or selectively pick and choose.

I had been thinking of just a subset of Real, but I think you're
right.  A full structure is better, even if some of it is identical.