big arrays

[Stephen Weeks]

One other thing that we need to keep in mind with array lengths of up
to valOf Int.maxInt is that computations of the number of bytes
required for an array can now overflow.  I'm not sure where we should
check for this.  Some possibilities:

* in the basis library code, using some kind of compiler hook to get
  bytesPerElt
* in the SSA code that Matthew was talking about for making array size
  computations explicit so that limit checks can be simplified