Provide an infallible ToJson trait in a separate crate

[dtolnay]

I would like an infallible JSON value conversion crate. Basically rustc_serialize::json::ToJson but for serde_json::Value.

With serde_json::to_value you get a fallible conversion that may fail for example because the input is a mutex that is poisoned, or because the input is a map with non-string keys, or for any number of other reasons. In some situations you want the type system to guarantee that the conversion cannot fail in those ways.

An infallible conversion would require its own custom derive. Something like:

#[derive(ToJson)]
struct S<T> {
    a: String,
    b: T,
}

// expands to
impl<T> ToJson for S<T> where T: ToJson {
    fn to_json(&self) -> serde_json::Value {
        let mut map = serde_json::Map::with_capacity(2);
        map.insert("a", self.a.to_json());
        map.insert("b", self.b.to_json());
        serde_json::Value::Object(map)
    }
}