Null<T> lost in type inference

``` haxe
class Node {
public function new(value:T, next:Node) {
}
}

class Main {
static function main() {
var i:Null = 1;
new Node(i, new Node(1, null));
}
}
```

generated AST (note the inner Node is a `Node`, not `Node>`):

```
[New:Node>]
Node>
[Cast:Null] [Const:Int] 1
[New:Node]
Node
[Const:Int] 1
[Const:Node] null
}
```

this is problematic for C# target where we have to generate a copy-cast right now