`_dyn` can make hxcpp fail

Declaring a class with both a method `foo` and a method `foo_dyn` makes compilation fail, because hxcpp adds its own `_dyn` versions, so it ends up declaring:

```cpp
class ... {
// ...
::Dynamic foo();
::Dynamic foo_dyn();
::Dynamic foo_dyn();
::Dynamic foo_dyn_dyn();
}
```