Global Opaque Xarray.

Lemma Xarray_update:
forall i b,
Zlength b = LBLOCKz ->
16 <= i < 64 ->
upd_Znth (i mod 16) (map Vint (Xarray b (Z.to_nat i)))
(Vint (W (nthi b) i))
= map Vint (Xarray b (Z.to_nat i+1)).