Add Parquet loader + from_parquet and to_parquet
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`pyarrow` 1.0.0 doesn't support some types in parquet, we'll have to bump its minimum version.

Also I still need to add dummy data to test the parquet builder.
2021-06-22T17:35:50Z
https://github.com/huggingface/datasets/pull/2537#issuecomment-866190768
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I had to bump the minimum pyarrow version to 3.0.0 to properly support parquet.

Everything is ready for review now :)
I reused pretty much the same tests we had for CSV
2021-06-24T13:13:05Z
https://github.com/huggingface/datasets/pull/2537#issuecomment-867626030
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2021-06-24T13:13:37Z
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Done !
Now we're still allowing pyarrow>=1.0.0, but when users want to use parquet features they're asked to update to pyarrow>=3.0.0
2021-06-30T16:30:23Z
https://github.com/huggingface/datasets/pull/2537#issuecomment-871555632
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Continuation of #2247 

I added a "parquet" dataset builder, as well as the methods `Dataset.from_parquet` and `Dataset.to_parquet`.
As usual, the data are converted to arrow in a batched way to avoid loading everything in memory.