use std::fill_n() to fill row data
This one surprised me, because you'd think a modern compiler could detect the pattern anyway, but it's good for ~2 percent in a tight loop which you can hit when catting giant amounts of data. Interestingly though I wonder if it's the type of thing to be using more. I had hoped we could just make most row-data filling "0" for defaults but that is a lot more work.