[Pitch] Aliased Span and Ref types

Hi all, I've written a proposal to introduce a family of Aliased*Span and Aliased*Ref types that provide the memory safety guarantees of the Span and Ref family of types, but with looser requirements around exclusivity, making them suitable for shared memory and interoperability with other languages. There's a lot of deep Law of Exclusivity and pointer-aliasing stuff in this proposal, but the APIs themselves are fairly straightforward mirrors of the *Span and *Ref types, albeit with small crit...