Transparent Mem.load.

Lemma load_None_obs:
forall (mc mf : mem) (f:memren)
(b1 b2 : block) chunk (ofs : Z)
(Hload: Mem.load chunk mc b1 ofs = None)
(Hf: f b1 = Some b2)
(Hinjective: forall b0 b1' b3 : block, f b0 = Some b3 -> f b1' = Some b3 -> b0 = b1')
(Hobs_eq: strong_mem_obs_eq f mc mf),
Mem.load chunk mf b2 ofs = None.