Add serde support feature

[IcanDivideBy0]

Hello,
I think serde support would be nice.

[IcanDivideBy0]

Specifically, I'm trying to match currencies with open exchange rates API:

#[derive(Debug, Clone, Deserialize)]
struct Response {
    base: Currency,
    rates: HashMap<Currency, f32>,
}

let app_id = "SECRET";
let uri = format!(
    "http://openexchangerates.org/api/latest.json",
    app_id
);
// response:
// {
//     base: "USD",
//     rates: {
//         AED: 3.672538,
//         AFN: 66.809999,
//         ...
//     }
// }

#[derive(Debug, Clone, Deserialize)]
struct Response {
    base: Currency,
    rates: RatesMap,
}

let response: Response = reqwest::get(uri).await?.json().await?;

So for this example, Currency should also derive Eq and Hash.

Would you consider such a PR with serde behind a feature flag ?

[zbrox]

Sounds great!