space profiling

[Matthew Fluet]

> As to the 64 bit integer support, for now just using IntInf's should be fine.
> It isn't like mlprof does  much.   

True.  But, we'll want them eventually, particularly for Position.int's in
files.

> As  to  the  code  in  the  program  being
> profiled, I would still probably vote (weakly) for doing this in C.  With the
> global nature of MLton, any ML code could cause changes in other things.

Vote for doing what in C?