WIP: Add a Share kind

`Share` implies that all *reachable* content is *threadsafe*.

Threadsafe is defined as "exposing no operation that permits a data race if multiple threads have access to a &T pointer simultaneously". (NB: the type system should guarantee that if you have access to memory via a &T pointer, the only other way to gain access to that memory is through another &T pointer)...

Fixes #11781