Ensure ``BFIndex`` is pickleable

It's currently not possible to pickle the ``BFIndex``, while the ``Index`` can be pickled.

```python
import pickle
from hnswlib import BFIndex

index = BFIndex(space="l2", dim=10)

with open("index.hnswlib", "wb") as f:
    pickle.dump(index, f)
```

It would be nice if both index types could be pickled, how hard would it be to support pickling for ``BFIndex``?