Support for initializing `enum` variants in `pin-init`

[dakr]

Support for initializing enum variants in pin-init

Currently, pin-init does not support initializing enum types or variants, which is limiting in cases where pinned data must be conditionally constructed. However, this requires a way to write the discriminant of an enum.

There has been prior discussion on this topic:

• 💬 Zulip discussion: Prior discussions for manually building an enum?
• 📜 RFC: Unsafe Set Enum Discriminants

---

Example

#[pin_data]
struct Foo {
    // ...
}

#[pin_data]
struct Bar {
    // ...
}

#[pin_data]
enum MyEnum {
    VariantA {
        #[pin]
        foo: Foo,
    },
    VariantB {
        #[pin]
        bar: Bar,
    },
}

#[pin_data]
struct Wrapper {
    #[pin]
    data: MyEnum,
    extra: u32,
}

impl Wrapper {
    fn new(init_data: impl PinInit<MyEnum>) -> impl PinInit<Self> {
        pin_init!(
            data <- init_data,
            extra: 42,
        )
    }
}