Change field order to drop Environment last

[acrrd]

This changes the order of the field to drop Environment after everything else.

The current code gets stuck, randomly but steadily, during the drop. What happened is that SessionPointerHolder held by the session is dropped after the Environment, and this seems to cause it to be blocked indefinitely.

This is an example code that shows that the program gets stuck. This happens with both ort 1.15 and 1.16. It happens with different models.

use ort::{
    environment::Environment,
    session::{Session, SessionBuilder},
};

pub struct Model {
    _session: Session,
}

impl Model {
    fn new() -> Self {
        let environment = Environment::builder().build().unwrap().into_arc();
        let session = SessionBuilder::new(&environment)
            .unwrap()
            .with_model_from_file("./model.onnx")
            .unwrap();

        Model {
            _session: session,
        }
    }
}

fn main() {
    Model::new();
}

With the proposed fix, it doesn't get stuck anymore. This could also be achieved by adding an env field after session in Model, but since Session already holds an Environment, it shouldn't be needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a1c19d) 51.88% compared to head (4c9e861) 52.30%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   51.88%   52.30%   +0.42%     
==========================================
  Files          13       13              
  Lines        1089     1084       -5     
==========================================
+ Hits          565      567       +2     
+ Misses        524      517       -7     

Files	Coverage Δ
src/session.rs	64.43% <ø> (ø)

... and 3 files with indirect coverage changes

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