Make `label_dict` conform (more) to Python's `dict` contract

Add methods to `label_dict`
- iterators `keys`, `values`, `items`
- `update(dict)`
- `label_dict(iter[k, v])`, `label_dict(label_dict)`
- `__contains__`

Also bump `pybind11` to latest and clean up `label_dict` implementation.

Addresses #1535 (in so far as possible)