Convert std::vec::raw::{init_elem, copy_memory} to methods, and remove aliasing &[] and &mut[]

The removal of the aliasing &mut[] and &[] from `shift_opt` also comes with its simplification.

The above also allows the use of `copy_nonoverlapping_memory` in `[].copy_memory` (I did an audit of each use of `.copy_memory` and `std::vec::bytes::copy_memory`, and I believe none of them are called with arguments can ever alias). This changes requires that `unsafe` code using `copy_memory` **needs** to respect the aliasing rules of `&mut[]`.