MLton RAM usage

[Henry Cejtin]

The point here (which is easily worked around) is that `free' memory is memory
which is currently un-used, not even for the disk cache.  Thus it is usually
(except right after a big program exits) near 0.
This part of the problem is easily worked around because you just add the
buffers and cached to free.  Note, I am really not convinced that this is
a good measure of how big MLton should get, but it does work around this
part of the problem.