big arrays [was Re: bug found]

[Matthew Fluet]

> > What would be your proposed object layout (or only arrays having top-bit-0)?
> 
> Header word
> 	           mark
> 	  31 30 29 28
> normal   1  0  0
> stack    1  1  0
> cont     1  1  1
> array    0  0  0

Why do we need to distinguish between stack's and cont's?  If the only
difference between stacks and conts is that the latter has used ==
reserved, then that's easily tested at any point where we need to
distinguish between the two (we'll already know that the object is a
stack/cont).  And, I don't even see where that distinction is used.