Implemented Decodable/Encodable for Cell and RefCell. Fixes #15395

Updated PR with fixme and test

Updated PR with fixme and test
auto merge of #15411 : mitchmindtree/rust/master, r=alexcrichton

I ran `make check` and everything went smoothly. I also tested `#[deriving(Decodable, Encodable)]` on a struct containing both Cell<T> and RefCell<T> and everything now seems to work fine.