Function build_codec 

fn build_codec<'a, B>(
    rows: impl IntoIterator<Item = &'a B>,
) -> Option<ColumnsCodec>
where
    B: Borrow<RowRef> + ?Sized + 'a,

Gather encoding statistics across rows and produce a codec from them.

Accepts anything that borrows as a RowRef, so it serves both the columnation-fed builders (which yield &Row) and the paged builders (which yield &RowRef straight out of a Column chunk).

Returns None when dictionary compression is disabled.