missing a way to provide access to private types through a macro

Subj. It could be useful with `@:genericBuild` for example:

``` haxe
@:genericBuild(MyMacro.build()) // macro returns `SugarTypeImpl` with proper type params
class SugarType {}

private class SugarTypeImpl {
// ...
}
```

We don't want to provide direct access to `SugarTypeImpl`, only through `SugarType` by returning it from its `@:genericBuild` macro.