@:structInit can be used to bypass final assignment

Putting `@:structInit` on a child class auto-generates a constructor that doesn't initialize the `final` fields of the base class, which seems unsound:

```haxe
function main() {
final child:Child = {s: ""};
trace(child.i); // undefined
}

class Base {
public final i:Int;

public function new(i:Int) {
this.i = i;
}
}

@:structInit class Child extends Base {
public final s:String;
}
```

If `Base` is itself a `@:structInit` class without an explicitly declared constructor, you get the compile-time error you would probably expect:

```
source/Main.hx:3: characters 23-30 : Object requires field i
```