Nested arrays have backwards shapes in `dtype`

[ExpHP]

use npy::NpyData;
use npy::Serializable;

#[derive(npy_derive::Serializable)]
#[derive(Debug, PartialEq)]
struct Array23 {
    field: [[i32; 3]; 2],
}

fn main() {
    println!("{}", <Array23 as Serializable>::dtype().descr());
}

[('field', '<i4', (3,2,)), ]

Observe that the outer dimension of 2 comes last in the output.

(Thankfully, saving a file still writes the correct number of total elements... it's just that, if you read the file in numpy, it'll turn into a 3x2 array)

A fix for this will be included in #15 since it completely replaces the Serializable trait.