Add support for Split.ALL
Honestly, I think we should fix some other issues in Split API before this change. E. g. currently the following will not work, even though it should:
```python
import datasets
datasets.load_dataset("sst", split=datasets.Split.TRAIN+datasets.Split.TEST)  # AssertionError
```

EDIT:
Actually, think it's OK to merge this PR because the fix will not touch this PR's code.
The title says it all.