arc should require send trait

[brson]

As currently defined arc only requires T: const but this would allow them to store local boxes, which would not be safe.

[eholk]

Is this for the shared arc or the exclusive arc? I thought the point of the const trait was that we could safely share even @ boxes.

[eholk]

Nevermind, I thought about it some more on the way home and remembered that @ boxes have a reference count which is mutable.