Arc support - feasible?

[the80srobot]

Looks like Arc<T> is in the "TBD" stage, as documented here.

@dtolnay, just wondering, have you learned of any reasons why it might be difficult to support? If the hope is still that it'd be straightforward and you haven't gotten around to it, I might take a stab at it.

(I'm trying to share a global clock object between Rust and C++ code.)