Add unstable Share trait

[P8L1]

Tracking issue: #156756

This adds an initial unstable Share trait for clone-as-alias types, as part of the 2026 ergonomic ref-counting project goal.

pub trait Share: Clone {
    fn share(&self) -> Self {
        Clone::clone(self)
    }
}

This PR adds a separate unstable feature gate:

#![feature(share_trait)]

and places the trait next to Clone in core::clone.

Implemented initial impls:

• impl<T: ?Sized> Share for &T
• impl<T: ?Sized, A: Allocator + Clone> Share for Rc<T, A>
• impl<T: ?Sized, A: Allocator + Clone> Share for Arc<T, A>
• impl<T> Share for std::sync::mpsc::Sender<T>
• impl<T> Share for std::sync::mpsc::SyncSender<T>

The PR deliberately does not add Share to the prelude.

r? @nikomatsakis
@rustbot label F-ergonomic_clones

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