Update to serde_json 0.9

[dtolnay]

I noticed that #561 merged recently and is using serde_json 0.8. The serde_json::Value enum is much nicer to work with in 0.9, including easy indexing value["address"]["city"] and a json! macro for constructing them in a nice way.

Release notes: https://github.com/serde-rs/json/releases/tag/v0.9.0

[killercup]

Shouldn't be too much work.

cc @emk

[sgrif]

If we're able to do a version range that supports both without much additional work that would be preferable