Use pretty-assertions and improve debug implementation of Key

[Luro02]

While writing tests and debugging #10, I had some trouble with the way keys and differences are displayed. To illustrate this, consider this code:

#[test]
fn test_showcase() {
    assert_eq!(
        vec![
            (
                Key::from_array(b"namespace_two"),
                Key::from_array(b"example_u16")
            ),
            (
                Key::from_array(b"namespace_one"),
                Key::from_array(b"example_i16")
            ),
        ],
        vec![
            (
                Key::from_array(b"namespace_one"),
                Key::from_array(b"example_u8")
            ),
            (
                Key::from_array(b"namespace_one"),
                Key::from_array(b"example_i8")
            ),
            (
                Key::from_array(b"namespace_one"),
                Key::from_array(b"example_u16")
            ),
            (
                Key::from_array(b"namespace_one"),
                Key::from_array(b"example_i16")
            ),
        ]
    );
}

Without pretty-assertions and the key debug impl it will be displayed as

Without pretty-assertions, but the binary string debug impl

with just pretty-assertions:

and this is how it looks like with both pretty-assertions and the binary string debug impl:

[lhemala]

Thanks for the addition!