Add `{into,from}_raw` to Rc and Arc

These methods convert to and from a `*const T` for `Rc` and `Arc` similar to the way they work on `Box`. The only slight complication is that `from_raw` needs to offset the pointer back to find the beginning of the `RcBox`/`ArcInner`.

I felt this is a fairly small addition, filling in a gap (when compared to `Box`) so it wouldn't need an RFC. The motivation is primarily for FFI.

(I'll create an issue and update a PR with the issue number if reviewers agree with the change in principle **Edit: done #37197**)

~~Edit: This was initially `{into,from}_raw` but concerns were raised about the possible footgun if mixed with the methods of the same name of `Box`.~~

Edit: This was went from `{into,from}_raw` to `{into,from}_inner_raw` then back to `{into,from}_raw` during review.