Un-deprecate `from_path_iter` or make `Iter` public

[askoretskiy]

For my project I need a function that reads ENV file and do not change the current process environment.

I found out dotenv::from_path_iter is doing exactly what is needed. Unfortunately it is marked as deprecated and it is recommended to use from_path + vars.

The problem is that from_path updates the environment but I need just to read it, I would modify the env myself.

I could have used Ok(Iter::new(File::open(path).map_err(Error::Io)?)) but Iter struct is private.

[Plecra]

Fyi, the deprecation warning has been removed

[askoretskiy]

Thank you @Plecra . Any plans for release?