Add support for enums to derive FromValue

[udoprog]

This allows for enums to derive FromValue, like this:

#[derive(Debug, PartialEq, Eq, FromValue)]
enum Proxy {
    Unit,
    Tuple(String),
    Struct { field: String },
}

let proxy = rune! { Proxy => r#"
fn main() {
    enum Proxy { Unit, Tuple(a), Struct { field } }
    Proxy::Unit
}
"#};

assert_eq!(proxy, Proxy::Unit);

let proxy = rune! { Proxy => r#"
fn main() {
    enum Proxy { Unit, Tuple(a), Struct { field } }
    Proxy::Tuple("Hello World")
}
"#};

assert_eq!(proxy, Proxy::Tuple(String::from("Hello World")));

let proxy = rune! { Proxy => r#"
fn main() {
    enum Proxy { Unit, Tuple(a), Struct { field } }
    Proxy::Struct { field: "Hello World" }
}
"#};

assert_eq!(
    proxy,
    Proxy::Struct {
        field: String::from("Hello World")
    }
);