Make fixints usable through serde field attributes instead of wrappers

[lachlansneff-parallel]

This changes the (thankfully not public, by shear accident), fixint module to be usable through the #[serde(with = ...)] field attribute instead of wrappers.

For example:

#[derive(Serialize, Deserialize)]
pub struct DefinitelyBE {
    #[serde(with = "crate::fixint::be")]
    x: u16,
}

[netlify]

✅ Deploy Preview for cute-starship-2d9c9b canceled.

Name	Link
🔨 Latest commit	7ee1e18
🔍 Latest deploy log	https://app.netlify.com/sites/cute-starship-2d9c9b/deploys/62d1af45395be80008305b2a

[jamesmunns]

Awesome, thanks @lachlansneff-parallel!