variantly 0.1.0

Derive associated functions for enum variants that are familiar from `std::option::Option` & `std::result::Result` such as `unwrap_or` or `and_then`.
Documentation 
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//! The example that is used in the README and other various documentation.
use variantly::Variantly;

#[derive(Variantly)]
enum Color {
    RGB(u8, u8, u8),
    HSV(u8, u8, u8),
    Grey(u8),
    FromOutOfSpace,
    #[variantly(rename = "darkness")]
    Black,
}

#[test]
fn example() {
    let color = Color::HSV(123, 45, 67);

    // boolean helper function for determining variant:
    assert!(color.is_hsv());
    assert!(!color.is_rgb());

    // Get inner values:
    let (h, s, v) = color.unwrap_hsv();
    assert_eq!((h, s, v), (123, 45, 67));

    // Single values don't require tuple destructuring:
    let color = Color::Grey(128);
    let value = color.unwrap_grey();
    assert_eq!(value, 128);

    // Alter inner value, only if hsv:
    let color = Color::HSV(111, 22, 33);
    let color = color.and_then_hsv(|(h, s, _)| (h, s, 100));
    assert_eq!(color.unwrap_hsv(), (111, 22, 100));

    // Safely unwrap with a fallback:
    let color = Color::RGB(255, 255, 0);
    let (r, g, b) = color.unwrap_or_rgb((0, 0, 0));
    assert_eq!((r, g, b), (255, 255, 0));
    // Since color is of the HSV variant, the default is not used.

    // Safely unwrap using the fallback
    let color = Color::FromOutOfSpace;
    let (r, g, b) = color.unwrap_or_rgb((0, 0, 0));
    assert_eq!((r, g, b), (0, 0, 0));

    // Convert into an Option
    let color = Color::RGB(0, 255, 255);
    let optional_rgb = color.ok_rgb();
    assert_eq!(Some((0, 255, 255)), optional_rgb);

    // Convert into a Result
    let color = Color::RGB(255, 0, 255);
    let result_rgb = color.ok_or_rgb("Error: This is not an RGB variant!");
    assert_eq!(Ok((255, 0, 255)), result_rgb);

    // Operations like this can also use their familiar `_else` versions:
    let color = Color::FromOutOfSpace;
    let result_rgb = color.ok_or_else_rgb(|| Some("This is a computationally expensive error!"));
    assert!(result_rgb.is_err());

    // The `#[variantly(rename = "darkness")]` attribute renames associated functions:
    let color = Color::Black;
    assert!(color.is_darkness())
}