Special characters in field names should be escaped

[Patryk27]

This code:

use serde::Serialize;

#[derive(Default, Serialize)]
struct Foo {
    #[serde(rename = "bar\"")]
    bar: String,
}

fn main() {
    println!("{}", serde_json::to_string_pretty(&Foo::default()).unwrap());
}

... returns:

{
  "bar\"": ""
}

... but Doku will document it in a wrong way:

{
  "bar"": "string"
}

[Patryk27]

(we already have the escaping logic implemented as a part of value_to_string.rs, so my rough guess is that we should just re-use it to escape all keys before passing them into the printer.)