Add an implementation of Encodable and Decodable for Rc. This will be needed to use Rc in place of @ in libsyntax.
Implement IterBytes for Rc<T>.
auto merge of #10658 : LeoTestard/rust/serialize-rc, r=cmr

Implement various traits (IterBytes and extra's Encodable and Decodable) for Rc<T> when T alreay implements the trait.