Rename UnsafeAtomicRcBox to UnsafeArc. Fixes #7674.
std: use ArcData rather than c_void in UnsafeArc.

This means that fewer `transmute`s are required, so there is less
chance of a `transmute` not having the corresponding `forget`
(possibly leading to use-after-free, etc).
auto merge of #8790 : huonw/rust/unsafearc, r=thestinger

`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.