Function parallel_columns_impl 

pub(crate) fn parallel_columns_impl(
    df_name: &Ident,
    row_name: &Ident,
    generics: &Generics,
    bounds: &[(TokenStream, TokenStream)],
    body: TokenStream,
) -> TokenStream

The parallel column fill of a companion: ParallelColumns with body as from_rows_parallel(rows), implemented only where bounds hold (see miniextendr_api::dataframe::ParallelColumns). Emitted only when miniextendr-api has rayon; pair it with from_rows_par_override.