Transparent Mem.alloc.

(*stronger version of val_at_alloc_1*)
Lemma val_at_alloc_3:
forall m m' lo hi  nb b ofs
(Halloc: Mem.alloc m lo hi = (m', nb))
(Hvalid: b <> nb),
Maps.ZMap.get ofs (Maps.PMap.get b (Mem.mem_contents m)) =
Maps.ZMap.get ofs (Maps.PMap.get b (Mem.mem_contents m')).