message: Improve readability of do_next implementation
diff: @@ -241,18 +241,20 @@ mod iter_support {
  // prepare return values
  let (key, value) = match item {
  Some(item) => item,
  - None => return SUCCESS,
  + None => return SUCCESS, // Return early without writing key. Empty key will later be treated as _no more element_.
  };
  +
  match write_region(ctx, key_ptr, &key) {
  - Ok(()) => SUCCESS,
  + Ok(()) => (),
  Err(Error::RegionTooSmallErr { .. }) => return ERROR_REGION_WRITE_TOO_SMALL,
  Err(_) => return ERROR_REGION_WRITE_UNKNOWN,
  };
  match write_region(ctx, value_ptr, &value) {
  - Ok(()) => SUCCESS,
  - Err(Error::RegionTooSmallErr { .. }) => ERROR_REGION_WRITE_TOO_SMALL,
  - Err(_) => ERROR_REGION_WRITE_UNKNOWN,
  - }
  + Ok(()) => (),
  + Err(Error::RegionTooSmallErr { .. }) => return ERROR_REGION_WRITE_TOO_SMALL,
  + Err(_) => return ERROR_REGION_WRITE_UNKNOWN,
  + };
  + SUCCESS
  }
  // if ptr is None, find a new slot.
  