Rename functions to read CSV

[etiennebacher]

Mentioned in #267 (comment)

This PR:

• lazy_csv_reader -> scan_csv
• csv_reader -> read_csv

For now, they only can be accessed with pl$ because I removed read_csv_, but if we still want to have this kind of high-level helper, we should rename it (maybe read_csv_pl()?)

Feel free to push directly here

[eitsupi]

Thank you for working on this!

I thought we should do this too, but when I looked at the py-polars implementation I realized that Python's polars.scan_csv is completely different from the current R's lazy_csv_reader.

That is, Python's scan_csv seems to map directly to Rust's scan_csv.
https://github.com/pola-rs/polars/blob/d40e403c81620f659244af20ce36470af8afe419/polars/polars-lazy/polars-plan/src/logical_plan/builder.rs#L206

[sorhawell]

actually py-polars uses also LazyCsvReader via here and via here

I have no idea what the difference between scan_csv and LazyCsvReader in rust-polars are. I was not aware of scan_csv in rust-polars.

[eitsupi]

Thank you for the updates! I think it is a good change for consistency.

Perhaps we can merge this since it seems to have passed the test?

[eitsupi]

actually py-polars uses also LazyCsvReader via here and via here

Oh, sorry. I must have misread the code.......