term: Replace manual Extend impl for Table — heartwood

There’s nothing more to say here.

         self.rows.push(Row::Header(row));
     }
     pub fn extend(&mut self, rows: impl IntoIterator<Item = [T; W]>) {
         for row in rows.into_iter() {
             self.push(row);
         }
     }
     pub fn is_empty(&self) -> bool {
         !self.rows.iter().any(|r| matches!(r, Row::Data { .. }))
     }

     }
 }
 impl<const W: usize, T: Cell> Extend<[T; W]> for Table<W, T> {
     fn extend<I>(&mut self, iter: I)
     where
         I: IntoIterator<Item = [T; W]>,
     {
         self.rows.extend(iter.into_iter().map(|row| {
             for (i, cell) in row.iter().enumerate() {
                 self.widths[i] = self.widths[i].max(cell.width());
             }
             Row::Data(row)
         }));
     }
 }
 #[cfg(test)]
 mod test {
     use crate::Element;