Add support for extern_spec for enum

[ranjitjhala]

The following now works

#[extern_spec]
#[flux::refined_by(b:bool)]
enum Option<T> {
    #[flux::variant(Option<T>[false])]
    None,
    #[flux::variant({T} -> Option<T>[true])]
    Some(T),
}

#[flux::sig(fn(x:Option<T>[true]) -> T)]
pub fn my_unwrap<T>(x: Option<T>) -> T {
    match x {
        Option::Some(v) => v,
        Option::None => never(0),
    }
}

pub fn test3() {
    let x = Option::Some(42);
    let y = my_unwrap(x);
    assert(y == 42);
}

[nilehmann]

One major comment, but overall it looks good.

I don't mind the hack (I considered myself). I was settling into something more sophisticated and expanding the macro to:

#[flux::extern(enum)]
mod _extern_option {
    type Option<T> = core::option::Option<T>;
    
    enum FakeOption<T> {
        #[flux::variant(Option<T>[false])]
        None,
        #[flux::variant((T) -> Option<T>[true])
        Some(T)
    }
}

but that requires a bit more thought and in the end it's equally hacky.

[nilehmann]

LGTM