Function keep_by 

fn keep_by(
    sel: &mut Vec<u32>,
    len: usize,
    nulls: Option<&NullBuffer>,
    p: impl Fn(usize) -> bool,
)

keep for a column with no values slice to walk: a bitmap, a variable offset array or a fixed stride, none of which a vector register helps with. The win here is only the closure being monomorphic rather than boxed.