Getting rid of rustc-serialize

[est31]

serde 1.0 has been around since over two years now, so I think one could think about removing rustc-serialize. I see two tasks:

• Removing the use as a dev-dependency in the tests of rust-url: Make the tests use serde_json instead of rustc_serialize #514
• Removing the use as a dev-dependency in the tests of idna: Move tests of idna to serde-json #518
• Removing the optional feature (for a future 2.0, see Tracking issue for rust-url 2.0 #463): url crate: remove rustc-serialize optional dependency #507

[est31]

@SimonSapin for idna, should I file a PR for the master or the 2.0 branch?

[SimonSapin]

Anything that is not a breaking change of public API is fine in either. I’ll merge them before publishing 2.0.0.

[nox]

Thanks for your work, @est31!