Field optionality in object intersections incorrectly depends on order.

[Minimal example](https://try.haxe.org/#F612a9c5):

```haxe
function main() {
var v:Optional & Mandatory = {};// succeeds
var v:Mandatory & Optional = {};// fails
}

typedef Optional = { final ?o:Int; }
typedef Mandatory = { final o:Int; }
```

Run the linked try.haxe.org minimal example and confirm that reversing the two object intersections changes whether the empty object is accepted. Then locate the Haxe compiler's handling of object intersections and optional versus mandatory fields; done means both declarations behave consistently and the regression is covered by an appropriate compiler test.

Written by the indexing model from the issue text.