function binding and native optional arguments

```haxe
class Main {
public static function main() {
var foo: (?a: Int, ?b: Int) -> Void = null;
foo(null, null); // throws on static targets

var bar = foo.bind(_, 0); // (?a: Null) -> Void
bar(null); // passes
}
}
```

I think in this case `bar` type should become `(a: Int) -> Void` instead as a default value for the native optional argument is lost.

To compensate stricter rules, maybe some construction can be used to specify default values in a binding:
```haxe
var bar = foo.bind(_ = 0, 0); // (?a: Null) -> Void
```

This might be handy not only to this specific issue, but in general too.