feat: implement Stream for RowCursor

[timvermeulen]

I only just now saw that #122 was closed as not planned, so no worries if you're not interested in merging this functionality 🙂

Summary

• Adds impl<T: RowOwned + RowRead> Stream for RowCursor<T>
• Removes the use of UnsafeCell from BytesExt using polonius-the-crab, which the above needed anyway

The mocked_select benchmark with validation=off has regressed 20-30% on my machine, but select_market_data/select_numbers/select_nyc_taxi_data all stayed the same, so I'm not entirely sure how significant this is.

It's worth noting that moving the fn poll_next logic out of async fn next has subtly changed the behavior of next, because the Next future doesn't track any state (other than if it has resolved) — every time RawCursor::poll_next wakes up the task, RowCursor::poll_next will first try to deserialize the next row using the available bytes before calling self.raw.poll_next. I'm not sure if this will matter in practice but if necessary I could move the self.raw.poll_next call to the start of the loop in RowCursor::poll_next.

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided so that we can include it in CHANGELOG later
• For significant changes, documentation in README and https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials

[serprex]

what returns this?

[timvermeulen]

RowCursor::next basically does, except that it immediately awaits it