ENH: Create a sequence type that acts like `map_to_object`

So instead of 

```cpp
template<>
struct to_object<my_favorite_object> {
    static py::owned_ref<> f(const my_favorite_object& v) {
        return sequence_to_list(v);
    }
};
```

You'd write 


```cpp
template<>
struct to_object<my_favorite_object> : public sequence_to_object<my_favorite_object> {};
```