Not_found when nesting @:generic type and field

```
class Main {
static function main() {
var k = new X();
}
}

@:generic
class X {
public function new() {
var k:String = test();
}

@:generic function test():C2 {
return null;
}
}
```

```
Fatal error: exception Not_found
Raised at file "type.ml", line 743, characters 36-45
Called from file "type.ml", line 765, characters 16-59
Called from file "codegen.ml", line 298, characters 40-64
```

Some debug output:

```
Add copied field test to X_Main.cl_ordered_fields
Make generic field: test_String on X
Check field test_String on X_Main
```

So the problem is that at the time of copying the fields we do not yet have the generic specialization test_String because that is only created once we type the call to it in X.new. Intuitively I would say that we should type the entire generic base class before we start making copies of its fields, but I'm pretty sure this is going to be headache material.