Serializing/Deserializing guides (+ caching)

The goal is to allow users to save/load the index to/from disk and re-use it. After this we can re-enable caching. This may require changes in outlines-core for serialization/deserialization of the index.

```python
generator = Generator(model, OutputType)
generator.to_disk("...")

generator = Generator.from_disk("...")
```

Open question: should we compress large indexes?