Tracking issue: Take advantage of new features in stable Rust

- [ ] [`needs_drop`](https://doc.rust-lang.org/nightly/std/mem/fn.needs_drop.html) Rust 1.21.
- [ ] [`NonNull<T>` ](https://doc.rust-lang.org/nightly/std/ptr/struct.NonNull.html)(raw pointer wrapper) Rust 1.25.  #434