Shelley genesis use serde json arbitrary precision

[ecioppettini]

I can't really explain why, but I've been having some weird issues with the deserialize_string_from_number from serde-aux.

It seems to work when I run the tests in this repo, but it fails when I use it from carp (but it didn't at some point, which is the weird part).

The function itself uses this code inside

    #[derive(Deserialize)]
    #[serde(untagged)]
    enum StringOrNumber {
        String(String),
        Number(i64),
        Float(f64),
    }

Which I think shouldn't work in the first place (serde-rs/serde#2661 (comment))

So I think it's better to just use https://github.com/serde-rs/json/blob/1faf3a1db661c7fb1f3a9a141efa30fb67e9ab3a/Cargo.toml#L70

I also think the current code doesn't do what the comments imply. Numbers get deserialized as strings if you put quotes around them in the json, in which case it is lossless, but otherwise they are just parsed as floats (or i64), which introduces an error anyway before making it a fraction. This probably doesn't matter as much since no one is really using these fields, but probably best to avoid the issue.

The only problem with this is that the Fraction type doesn't support parsing scientific notation, so I did implement that manually here.