[MLton] ref flattening enabled

[Henry Cejtin]

I am ashamed to say that I wasn't thinking so much of space safety as much as
of space efficiency, but ...

Is 2 really doable?  Surely it would have to be some kind  of  approximation,
right?   I.e.,  it  would  take  some kind of must-alias analysis (i.e., this
variable must keep this object alive) to get this right.

For efficiency, I guess that if Dan Wang's box primitive went in then I could
at least forcibly stop the flattening (but not force the flattening).