left_child_with_depth and right_child_with_depth

[ralphtheninja]

If it turns out that left_child_with_depth and right_child_with_depth can be private, there's a few things we could change after that

1. If they aren't used internally (which I suspect they aren't), then we could merge them, so body of left_child_with_depth is moved to left_child (and similarly for right_child)
2. The depth == 0 check is no longer needed, since this only happens for even indices

So

pub fn left_child_with_depth(i: usize, depth: usize) -> Option<usize> {
  if is_even(i) {
    None
  } else if depth == 0 {
    Some(i)
  } else {
    Some(index(
      depth - 1,
      offset_with_depth(i, depth) << 1,
    ))
  }
}

pub fn left_child(i: usize) -> Option<usize> {
  left_child_with_depth(i, depth(i))
}

Could be

pub fn left_child(i: usize) -> Option<usize> {
  if is_even(i) {
    None
  } else {
    let d = depth(i)
    Some(index(
      d - 1,
      offset_with_depth(i, d) << 1,
    ))
  }
}