Switch from Row to_datum_iter/packer to copying

[antiguru]

At the moment, there are many places where we do the following:

let row: &Row = ...;
let mut packer = other_row.packer();
packer.extend(row.to_datum_iter());

This takes all datums from row and pushes them into other_row by means of decoding and encoding each individual item. There is an obvious way to make this faster based on the observation that the decoding/encoding is idempotent: We can instead just copy the bytes backing the datums. This probably means to skip some ToDatumIter::to_datum_iter calls since we're lacking specialization.

[antiguru]

It might be worth going though all uses of RowPacker::extend, because there are several that append a whole row (or chaining a row with another iterator.)

[sdht0]

Thanks @antiguru. I opened #28139.