iterate lazy rows?

[piever]

Tables.jl uses a lazy row type (basically a custom type that has the original table and the index of the row) instead of materializing the whole NamedTuple. I was wondering whether we should do the same here. It has the advantage that map(i -> i.x+i.y, t) would already be efficient (at least in the non distributed case) without having to explicitly having to type map(i -> i.x+i.y, t, select = (:x, :y)). We could then drop our collect_columns code and use the mechanism at Tables (see here).