feature: add conditional support for serde

[hackaugusto]

This adds conditional support for serde. It allow serializing/deserializing the field elements, which is useful for downstream applications, for instance, to save test data.

The change is behind a feature flag to avoid impacting compilation times.

[hackaugusto]

Simple test for the output format:

use serde::{Deserialize, Serialize};
use serde_json::json;
use winter_math::fields;

#[derive(Serialize, Deserialize)]
struct Data {
    d0: fields::f64::BaseElement,
    d1: fields::f62::BaseElement,
    d2: fields::f128::BaseElement,
}

fn main() {
    let v = Data {
        d0: 1234123u64.into(),
        d1: 1234123u64.into(),
        d2: 1234123u64.into(),
    };

    println!(
        "{}",
        serde_json::to_string(&v).expect("json serialization failed")
    );
}

The code using into and from annotations outputs the values are converted from montgomery form to human readable numbers:

{"d0":1234123,"d1":1234123,"d2":1234123}

In contrast, without them:

{"d0":5300517923007285,"d1":1483409960899751059,"d2":1234123}

[irakliyk]

Looks good! Thank you!