Derive unsafe

[greyblake]

This pull request brings derive_unsafe which allows to derive any arbitrary trait (just the way one would expect it to be with regular derive(..) without nutype

It's expected to address the following open issues:

• Enable utoipa::ToSchema #216
• Relax the rules around derive #191

Why it's called derive_unsafe?

It's called derive_unsafe because it enables to derive any traits that nutype is not aware of.
So it is developer's responsibility to ensure that the derived traits do not create a loophole to bypass the constraints.
As the rule of thumb avoid using derive_unsafe with traits that:

• Create a new instance of the type
• Mutate the value

Example

use derive_more::{Deref, DerefMut};
use nutype::nutype;

#[nutype(
    derive(Debug, AsRef),
    derive_unsafe(Deref, DerefMut),
    validate(greater_or_equal = 0.0, less_or_equal = 2.0)
)]
struct LlmTemperature(f64);

fn main() {
    let mut temprature = LlmTemperature::try_new(1.5).unwrap();

    // This is not what nutype is designed for!
    *temprature = 2.5;

    // OH no, we've just violated the validation rule!
    assert_eq!(temprature.as_ref(), &2.5);
}

TODO

• Update CHANGELOG
• Update docs in lib.rs
• Update docs in README