variantly 0.4.0

Derive helper methods 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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mod helper;
use helper::{
    TestEnum,
    TestEnum::{Int, Unit},
};

#[test]
fn single_value_tuple() {
    // Match
    assert_eq!(Int(123).int_mut(), Some(&mut 123));

    // Non-Match
    assert_eq!(Unit.int_mut(), None);
}

#[test]
fn multi_value_tuple() {
    // Match
    assert_eq!(
        TestEnum::new_tuple(123).tuple_mut(),
        Some((&mut "123".into(), &mut 123))
    );

    // Non-Match
    assert_eq!(Unit.tuple_mut(), None);
}