Switch from rustc-serialize to serde

[matthiasbeyer]

Serde is where the things happen today, so it would be awesome to switch to serde here.

[kstep]

I'd wait for proc_macro to stabilize (it should be stabilized in Rust 1.15.0), so we can use stable Rust.

[tomprince]

Now that 1.15 is out with proc_macro, I've started on an implementation here. There are still a few helpers that should be moved somewhere more sensible.

I'm not sure if that implementation is compatible with the current output format ... the serialization isn't used internally, so I'm not sure what the intended usage is.