Upper/lower diagonal subsets

[ghost]

I know there already exist DiagOffset, but how about these methods for Matrix?

fn upper_diag_inds(nrows: usize, ncols: usize) -> Vec<[usize; 2]> {
  (0..nrows)
    .enumerate()
    .flat_map(|(iter_ind, row_ind)| {
      (iter_ind..ncols).map(move |col_ind| {
        [row_ind, col_ind]
      })
    }).collect()
}

fn lower_diag_inds(nrows: usize, ncols: usize) -> Vec<[usize; 2]> {
  (0..nrows)
    .enumerate()
    .flat_map(|(iter_ind, row_ind)| {
      (0..iter_ind + 1).map(move |col_ind| {
        [row_ind, col_ind]
      })
    }).collect()
}

• Playground link

I think these would be useful for #144 also.