Improve raw Box conversions
This PR has two goals:
- Reduce use of `mem::transmute` in `Box` conversions
I understand that `mem::transmute`-ing non `#[repr(C)]` types is implementation-defined behavior. This may not matter within the reference implementation of Rust, but I believe it's important to remain consistent. For example, I noticed that `str::from_utf8_unchecked` went from using `mem::transmute` to using pointer casts.
- Make `Box` pointer conversions more straightforward regarding `Unique`