[MLton] 64-bit pointers

[Matthew Fluet]

> BTW, I've drifted over to the belief that simple 64bit
> pointers are best. Since amd64 is backwards compatible,
> there's no real problem for applications needing << 4GB.
> Just compile your program for i386 instead of amd64.
>
> I doubt that applications needing 4GB to 32GB will last
> very long in the history of computer evolution.

The typical ML heap is predominately pointers, so naively going from 
32-bit pointers to 64-bit pointers will almost double your memory 
requirements.  So it is the programs needing close to 4GB memory that 
suffer, because they'll need close to 8GB.