Optimize structures with one element to transparent

All structures with only one field should be unpacked. This would allow to produce faster code.

Location : in `typed_rust` after the lifetime analysis pass

Exemple :
```rust

struct MyNum {
     x : i32,
}

fn main() {
    let y = MyNum { x : 3 };
    
    let val = y.x;
}
```

The code above is equivalent to :

```rust
fn main() {
    let y = 3;
    let val = y;
}
```


