feat: return Row from iter_scan instead of mutating in-place

[imor]

This PR changes the iter_scan method signature from fn iter_scan(&mut self, row: &mut Row) -> Result<Option<()>, E> to fn iter_scan(&mut self) -> Result<Option<Row>, E>. The new signature is better in that the user is forced to return a Row instead of updating the incoming row in-place, which they can forget.

[burmecia]

The original design was to return a Row in iter_scan, but there was a performance issue with that design (see details #46), so we changed to use a borrowed Row in #48. IMO, we shouldn't cover this user negligence as it is the basic interface requirement, and can be easily identified.

[imor]

Oh I see, reusing the row saves some allocations because the Vecs in the row don't need to grow again for each new row. Closing for now.