?Sized bounds for rc::Weak::as_ptr and friends

[CAD97]

Specifically, this relaxes the bounds on rc::Weak::as_ptr, into_raw, and from_raw from impl<T> Weak<T> to impl<T: ?Sized> Weak<T>.

This is a follow-up to #73845, which did the impl work required to be able to relax these bounds.

IIUC, this technically stabilizes that "something like size_of_val_raw" is possible to do. However, I believe the part of the functionality required by these methods here -- specifically, getting the size of a pointee from a pointer where it may be dangling iff the pointee is Sized -- is uncontroversial enough to stabilize these methods without a way to implement them on stable Rust.

ATTN: This changes (relaxes) the (input) generic bounds on stable fn!

[rust-highfive]

r? @sfackler

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[CAD97]

...whoops, this apparently isn't quite possible yet. Closing for now.