Naming & minor type adjustments to std::unstable::sync::UnsafeAtomicRcBox

`UnsafeAtomicRcBox` &rarr; `UnsafeArc` (#7674), and `AtomicRcBoxData` &rarr; `ArcData` to reflect this.

Also, the inner pointer of `UnsafeArc` is now `*mut ArcData`, which avoids some transmutes to `~`: i.e. less chance of mistakes.