Make Deserializer public

[Pzixel]

What did you implement:

This change makes Deserializer type public. The motivation behind this is that from_env() function is too limiting and doesn't allow any control over the deserialization process.

For example consider this program:

#[derive(serde::Deserialize, Debug)]
pub struct MyEnvConfig {
    pub my_super_arr: [i32; 1],
}

// export my_super_arr=loremIpsum && cargo run
fn main() -> Result<(), Box<dyn std::error::Error>>{
    let env_config: MyEnvConfig = envy::from_env()?;
    Ok(())
}

It will exit with an error code and text

Error: Custom("invalid type: string "loremIpsum", expected an array of length 1").

However if we wrap the deserializer (which we can thanks to https://github.com/dtolnay/path-to-error ) we can get much more information about what happened:

#[derive(serde::Deserialize, Debug)]
pub struct MyEnvConfig {
    pub my_super_arr: [i32; 1],
}


fn main() -> Result<(), Box<dyn std::error::Error>>{
    let mut jd = envy::Deserializer::new(std::env::vars(), None);
    let env_config: MyEnvConfig = serde_path_to_error::deserialize(jd)?;
    Ok(())
}

Will print:

Error { path: Path { segments: [Map { key: "my_super_arr" }] }, original: Custom("invalid type: string "loremIpsum", expected an array of length 1") }

Now this is much better, because I can check the my_super_arr value.

I made this change specifically to be able to wrap into serde_path_to_error, because I have more than 100 env variables and some custom types do not mention name of the field that they failed to parse. For example primitive_types::H160 type says "expected string of length 40 but got 37". As you can imagine checking all the env strings for exact length is a very hard task.

But I think there are many other cases where wrapping the deserializer is desirable. Therefore, this type should be public

How did you verify your change:

I used my fork in my app to see if it works correctly

What (if anything) would need to be called out in the CHANGELOG for the next release:

I updated the CHANGELOG, but feel free to edit it whatever you please as I enable the "Allow edits by maintainers" checkbox.