Serialization

I added `save` and `load` method to serialize/deserialize a dataset object in a folder.
It moves the arrow files there (or write them if the tables were in memory), and saves the pickle state in a json file `state.json`, except the info that are in a separate file `dataset_info.json`.

Example:

```python
import nlp

squad = nlp.load_dataset("squad", split="train")
squad.save("tmp/squad")
squad = nlp.Dataset.load("tmp/squad")
```

`ls tmp/squad`
```
dataset_info.json squad-train.arrow state.json
```

`cat tmp/squad/state.json`
```json
{
  "_data": null,
  "_data_files": [
    {
      "filename": "squad-train.arrow",
      "skip": 0,
      "take": 87599
    }
  ],
  "_fingerprint": "61f452797a686bc1",
  "_format_columns": null,
  "_format_kwargs": {},
  "_format_type": null,
  "_indexes": {},
  "_indices": null,
  "_indices_data_files": [],
  "_inplace_history": [
    {
      "transforms": []
    }
  ],
  "_output_all_columns": false,
  "_split": "train"
}
```

`cat tmp/squad/dataset_info.json`
```json
{
  "builder_name": "squad",
  "citation": "@article{2016arXiv160605250R,\n       author = {{Rajpurkar}, Pranav and {Zhang}, Jian and {Lopyrev},\n                 Konstantin and {Liang}, Percy},\n        title = \"{SQuAD: 100,000+ Questions for Machine Comprehension of Text}\",\n      journal = {arXiv e-prints},\n         year = 2016,\n          eid = {arXiv:1606.05250},\n        pages = {arXiv:1606.05250},\narchivePrefix = {arXiv},\n       eprint = {1606.05250},\n}\n",
  "config_name": "plain_text",
  "dataset_size": 89789763,
  "description": "Stanford Question Answering Dataset (SQuAD) is a reading comprehension dataset, consisting of questions posed by crowdworkers on a set of Wikipedia articles, where the answer to every question is a segment of text, or span, from the corresponding reading passage, or the question might be unanswerable.\n",
  "download_checksums": {
    "https://rajpurkar.github.io/SQuAD-explorer/dataset/dev-v1.1.json": {
      "checksum": "95aa6a52d5d6a735563366753ca50492a658031da74f301ac5238b03966972c9",
      "num_bytes": 4854279
    },
    "https://rajpurkar.github.io/SQuAD-explorer/dataset/train-v1.1.json": {
      "checksum": "3527663986b8295af4f7fcdff1ba1ff3f72d07d61a20f487cb238a6ef92fd955",
      "num_bytes": 30288272
    }
  },
  "download_size": 35142551,
  "features": {
    "answers": {
      "_type": "Sequence",
      "feature": {
        "answer_start": {
          "_type": "Value",
          "dtype": "int32",
          "id": null
        },
        "text": {
          "_type": "Value",
          "dtype": "string",
          "id": null
        }
      },
      "id": null,
      "length": -1
    },
    "context": {
      "_type": "Value",
      "dtype": "string",
      "id": null
    },
    "id": {
      "_type": "Value",
      "dtype": "string",
      "id": null
    },
    "question": {
      "_type": "Value",
      "dtype": "string",
      "id": null
    },
    "title": {
      "_type": "Value",
      "dtype": "string",
      "id": null
    }
  },
  "homepage": "https://rajpurkar.github.io/SQuAD-explorer/",
  "license": "",
  "post_processed": {
    "features": null,
    "resources_checksums": {
      "train": {},
      "train[:10%]": {}
    }
  },
  "post_processing_size": 0,
  "size_in_bytes": 124932314,
  "splits": {
    "train": {
      "dataset_name": "squad",
      "name": "train",
      "num_bytes": 79317110,
      "num_examples": 87599
    },
    "validation": {
      "dataset_name": "squad",
      "name": "validation",
      "num_bytes": 10472653,
      "num_examples": 10570
    }
  },
  "supervised_keys": null,
  "version": {
    "description": "New split API (https://tensorflow.org/datasets/splits)",
    "major": 1,
    "minor": 0,
    "nlp_version_to_prepare": null,
    "patch": 0,
    "version_str": "1.0.0"
  }
}
```

Comments:
I've added save/load for dataset dicts.

I agree that in the future we should also have a way to save indexes too, and also the in-place history of transforms.

Also I understand that it would be cool to have the load function directly at the root of the library, but I'm not sure this should be inside `load_dataset` that loads dataset scripts and data from the dataset repository. Maybe something like `load_from_disk` ?

Yes `load_from_disk` and `save_to_disk`  could work as well.

I renamed save/load to save_to_dick/load_from_disk, and I added `nlp.load_from_disk`

`nlp.load_from_disk` can load either a Dataset or a DatasetDict.

Awesome! Let's add them to the doc and we're good to go!