[MLton] an analysis to flatten refs into tuples

[Stephen Weeks]

> >>With the ref flattening and the bit packing optimizations, I'm really 
> >>tempted to write a naive Scheme to ML source to source translator, and see 
> >>how well MLton does with the output
...
> The ref flattening would also be important.. so I can represent a cons cell 
> as a tuple of refs. 

Good point.  The other nice thing about the (not-yet-existent, keep in
mind) ref flattening is that it runs on our SSA IL after closure
conversion, and hence will flatten ref cells into closure records,
which seems to arise with many idioms.