bug found

[Henry Cejtin]

Note there is a trivial way to get bigger arrays, but it would make
Array.length slightly more expensive: you just have a different header
(using an extra word) for big ones.  The sub and update and all of those
would not change (although the cost of the runtime check on subscript out of
bounds would go up).  Not really ok because of the bounds check, but I'm
trying to see if I can come up with something more clever.
Either way .5 gig is bad but not horrible.