space profiling

[Stephen Weeks]

> It makes the computation of the right bin just a little more complicated.
> It might also give "incorrect" results, where time profile ticks for
> blocks get shifted forward or backwards.  Probably o.k. for space
> profiling, because there would be enough "buffer" space (i.e., setting
> everything up for the allocation and the tick) between the start of a
> block and the occurence of the tick, and likewise between the tick and the
> end of the block.

That's what I was thinking.  32 bit bins for time and 64 bit bins for space.

It's a minor change to mlprof (once we get Int64 added to MLton) to make it
handle both formats.