[MLton] MinGW done

[Stephen Weeks]

> I think it makes sense to merge the refactored runtime and Basis Library 
> into HEAD in the near future. 

That seems like a good idea to me.  It's where most/all development is
going on and it's stable enough to start living with on x86-linux.
Hopefully the other platforms will follow soon after the merge.  Plus,
merging will save Matthew work in tracking changes to the trunk, and
cut down on the chance for error there.

Is there any reason why it shouldn't happen now?