Move winapi support into a new unstable crate

[KodrAus]

For #113

Move the winapi support into a new unstable crate that's dedicated to UUID conversions:

pub trait IntoUuid {
    fn into_uuid(self) -> Uuid;
}

pub trait FromUuid {
    fn from_uuid(uuid: Uuid) -> Self;
}

We can then implement this trait for winapi's Guid and for the windows-rs Guid.

This will move our last remaining unstable public dependency out of the uuid crate so it can be stabilized.

[kinggoesgaming]

Are these traits are to be in uuid? I might work on Tuesday

[KodrAus]

@kinggoesgaming hey! 👋

Ah these traits are specifically for a new crate, called uuid-convert or something like that. That way we can remove winapi from uuid itself and anybody that wants to convert UUIDs can use this new unstable library.

The goal for doing this is so we don’t end up with winapi’s unstable API in uuid’s public one so we can safely stabilize uuid.

[KodrAus]

It's probably just as well we're looking to move our GUID support out, because I think the unconditional bitflipping we do is actually invalid. If I call CoCreateGuid and then pass the results to Uuid::from_guid I should end up with a V4 (random) UUID. But I don't, unless I don't flip the integers:

let mut guid = guiddef::GUID {
        Data1: Default::default(),
        Data2: Default::default(),
        Data3: Default::default(),
        Data4: Default::default(),
    };

    unsafe {
        CoCreateGuid(&mut guid as *mut _);
    }

    let uuid = Uuid::from_guid(guid);

    // This fails on `main`
    assert_eq!(Some(Version::Random), uuid.get_version());
}

[KodrAus]

The original PR that added GUID support (#336) was correct, and we regressed in #345.