Fast dataset iter
Use `pa.Table.to_reader` to make iteration over examples/batches faster in `Dataset.{__iter__, map}`

TODO:
* [x] benchmarking (the only benchmark for now - iterating over (single) examples of `bookcorpus` (75 mil examples) in Colab is approx. 2.3x faster)
* [x] check if iterating over bigger chunks + slicing to fetch individual examples in `_iter` yields better performance


