Use C layout for `ArcInner` and `RcInner`

I'll check to see if anyone points out any issues after I take a nap, otherwise I would say this is good to merge. Both `cargo test` and `miri` are happy even with `-Z randomize-layout` after this change _Sidenote: It would be good to get CI running with at least `cargo test` and `miri` since this uses `unsafe`_