Consider moving `objc2_foundation` into the `objc2` crate

[madsmtm]

There's a lot of places in objc2_foundation that would benefit from being able to implement From for specific Ids:

• NSArray::from_vec
• NSArray::into_vec
• NSData::from_vec
• NSDictionary::from_keys_and_objects
• NSString::from_str
• And so on...

Same goes for traits like Default.

But unfortunately, we can't create these implementations because Id is defined in another crate, see RFC 2451. I tried creating a helper trait FromId for this purpose, but that doesn't work either:

pub trait FromId<T, O: Ownership> {
    fn from_id(obj: Id<T, O>) -> Self;
}

impl<T, O: Ownership, X: FromId<T, O>> From<Id<T, O>> for X {
    fn from(obj: Id<T, O>) -> Self {
        Self::from_id(obj)
    }
}

// Errors with:
// note: conflicting implementation in crate `core`:
// - impl<T> From<T> for T;
// note: downstream crates may implement trait `FromId<_, _>` for type `Id<_, _>`

Though doing it for Default would:

// In objc2:

pub trait IdDefault: Sized {
    type Ownership: Ownership;
    fn id_default() -> Id<Self, Self::Ownership>;
}

impl<T: IdDefault> Default for Id<T, T::Ownership> {
    fn default() -> Self {
        T::id_default()
    }
}

// In objc2_foundation:

impl IdDefault for NSString {
    type Ownership = <NSString as INSObject>::Ownership;
    fn id_default() -> Id<Self, Self::Ownership> {
        <NSString as INSObject>::new()
    }
}

So possible solutions:

• Move objc2_foundation to objc2::foundation (under a feature flag)
• Move Id to objc2_foundation?

[madsmtm]

Might also be relevant to move most of the implementation of objc2_exception into objc2.

It currently uses an Exception type that doesn't implement Encode and Message, and it links manually to libobjc. Of course, these could be alleviated by introducing objc2_encode and objc2_sys as dependencies of it.

But it also returns *mut Exception instead of the correct Id<Exception, Shared>, and the only way to fix that is by moving the implementation to objc2.

[madsmtm]

I think we may move the objc2_exception implementation at some point, but the objc2_foundation separation should stay; it is fundamentally separate from the objc2 crate, and having these limitations in our own crate encourages us to create helper traits (like DefaultId and such, see #41) that can be used by all downstream crates.

[madsmtm]

I asked on the forum whether this is possible, we'll see if someone comes up with something; otherwise people will just have to import FromId/IntoId and use from_id/into_id manually

[madsmtm]

I'm not gonna move objc2_foundation, so I'll close this issue. Whether to use the approach in #48 or #50 can be determined in the PRs themselves.