write categorical to csv without casting

[ritchie46]

First will fix upstream: jorgecarleitao/arrow2#508

[ghuls]

Would it be hard to read certain columns of a CSV file directly to a Categorical?

[ritchie46]

Would it be hard to read certain columns of a CSV file directly to a Categorical?

We'd need to write a builder. Currently we only create them from iterators.

[ritchie46]

@ghuls it is quite complicated. Every thread must synchronize the string cache and later in rechunking we also need to do work to keep the catergories in sync.