Remove rustc_serialize

[Xanewok]

Originally created by @jonathandturner (rust-dev-tools/rls-analysis#60):

We still have

rustc-serialize = "0.3"

In the Cargo.toml. This is being deprecated, iirc.

[Xanewok]

Originally replied by @Enet4 (rust-dev-tools/rls-analysis#60 (comment)):

When migrating this (and rls_data) to serde, should the exact same JSON schema be retained? It seems that the default of serializing a PathBuf to JSON was an array of numbers, whereas serde would expect a string by default.

[Xanewok]

Originally replied by @nrc (rust-dev-tools/rls-analysis#60 (comment)):

@Enet4 we can't remove rustc_serialise from rls_data because it is used by rustc. I'm not actually sure what we use rustc_serialize for in the RLS, if it is for anything client-facing, then we'll need to maintain the JSON format.

[jabagawee]

I'd love to pick this up, but I'm trying to understand the context of this change. Adding more #[cfg()] statements is pretty trivial, but rustc emits data serialized with the rustc-serialize crate, which mean the tests won't work with the serde deserializer.

What is the goal of adding serde to rls-analysis? Can it be used in any circumstance, given that parts of rustc still use the format specified by rustc-serialize?

The minimal implementation can be seen at e100664.