rustup.

This commit switches to the external `rustc-serialize` crate for
decoding, which causes a breaking change. Namely, instead of using
`deriving(Decodable)`, you will now have to use
`deriving(RustcDecodable)`. Moreover, you must now link to the
`rustc-serialize` crate instead of `serialize`. So change this:

    extern crate serialize;

to this:

    extern crate "rustc-serialize" as rustc_serialize;

[breaking-change]
0.6.19