Auto merge of #23333 - oli-obk:slice_from_raw_parts, r=alexcrichton

Auto merge of #23333 - oli-obk:slice_from_raw_parts, r=alexcrichton

at least that's what the docs say: http://doc.rust-lang.org/std/slice/fn.from_raw_parts.html

A few situations got prettier. In some situations the mutability of the resulting and source pointers differed (and was cast away by transmute), the mutability matches now.