Get an IterableDataset from a map-style Dataset
This is useful to leverage iterable datasets specific features like:
- fast approximate shuffling
- lazy map, filter etc.

Iterating over the resulting iterable dataset should be at least as fast at iterating over the map-style dataset.

Here are some ideas regarding the API:
```python
# 1. 
# - consistency with load_dataset(..., streaming=True)
# - gives intuition that map/filter/etc. are done on-the-fly
ids = ds.stream()

# 2.
# - more explicit on the output type
# - but maybe sounds like a conversion tool rather than a step in a processing pipeline
ids = ds.as_iterable_dataset()
```