Make these convenient functions even more convenient:
1. Change `object_get_model()` to `object_get_model_num()` and `object_get_model_instance()` to `object_get_model_instance_num()` to more accurately reflect what they do
2. Add new `object_get_model()` and `object_get_model_instance()` functions to return `polymodel*` and `polymodel_instance*` values.