[r2r] enum from variant derive macro

[borngraced]

EnumFromVariant is very useful for generating From<T> trait from one enum variant to another. Currently, this crate can only convert enum variant with only some basic inner type such as String, and Enum type just like the example below. Can not be used for tuple, struct etc for now.

// E.G, this converts from whatever Bar is to FooBar::Bar(String) and
// whatever Foor to FooBar::Foo(Foo)
#[derive(Debug, EnumFromVariant, PartialEq, Eq)]
pub enum FooBar {
    #[enum_from_variant("Bar")]
    Bar(String),
    #[enum_from_variant("Foo")]
    Foo(Foo),
}

#[derive(Debug, Display, PartialEq, Eq)]
pub enum Bar {
    Bar(String),
}

#[derive(Debug, Clone, Display, PartialEq, Eq)]
pub enum Foo {
    Foo(String),
}

#[test]
fn test_from_variant() {
    let bar = Bar::Bar("Bar".to_string());
    assert_eq!(FooBar::Bar("Bar".to_string()), bar.into());

    let foo = Foo::Foo("Foo".to_string());
    assert_eq!(FooBar::Foo(foo.clone()), foo.into());
}

[sergeyboyko0791]

Awesome feature 💯
First review iteration

[borngraced]

@sergeyboyko0791 @shamardy this pr is ready for another round of review

[sergeyboyko0791]

Next review iteration :)

[sergeyboyko0791]

Thank you for the fixes!
Next iteration

[sergeyboyko0791]

Last review notes 🙂

[sergeyboyko0791]

A few suggestions only

[sergeyboyko0791]

One minor non-critical suggestion that may make the code more readable 🙂

[sergeyboyko0791]

Re-approve!

[shamardy]

LGTM!