Drop Serde support

[dtolnay]

trafficstars

As of serde_json 0.9 it provides its own stable macro_rules json! macro:

https://docs.serde.rs/serde_json/macro.json.html

Some neat things about it:

• macro_rules, works great on stable Rust
• no parens needed around expressions
• negative numbers work
• allows interpolation of map keys, not just values
• allows optional trailing commas

Unless there are some advantages of this library over that json! macro, let's drop Serde support and have this library be for rustc-serialize only.