[MLton] MLton calling convention and closure conversion

[Stephen Weeks]

> I would agree, and there is no hard and fast rule I can think 
> of to properly choose when to stack and when not.

The only thing I know of is to restrict to objects whose size is known
at compile time.  That, combined with the necessary lifetime
restriction, combined with the runtime-system complexities of stack
allocation makes me think that it is not a worthwhile road to go down.