MLton RAM usage

[Henry Cejtin]

You can get `free' memory in the sense of memory no one is using (not even
the buffer cache), but it is usually very very low (on my machine right now
it is 7 meg, and nothing is really running on it).  You can also just add
to that all the memory in the disk cache, but this number also is usually
too low (I don't know quite what the reason is).  The problem is that you
can't tell what other processes are really doing.  One possibility would be
to try to keep growing until you see that you are swapping all the time.
I don't really know of a good solution and for now I would just stick with
what we have, modulo fixing this problem of saved heaps and also not dieing
if you ask for more memory below maxHeap and the mmap fails.