introduce a magical KindMimic type

[thestinger]

Closes #9509

[thestinger]

cc @nikomatsakis

[thestinger]

This is a working solution to the problem for now. I've marked the struct with the #[unstable] stability marker so it can be revised later if more is needed. This should probably be split into separate type per-kind (SendMimic, FreezeMimic) because it's not going to make sense for Sized.

[nikomatsakis]

This seems like a sensible approach. I had originally thought to couple the pointer and the kind translations -- that is, replace the *T that appears in Rc<T> with something like IndirectPtr<T>, that would be a *T but would be specially understood by the kind checker in a similar fashion to KindMimic. This seemed like it would fit in with Raw<T> as a possible future replacement for *T. But I think I prefer your solution, since it decouples the representation of the pointer from the type checker concerns. Overall I am not pleased that smart pointers require such a deep understanding of language concerns, but I guess this is somewhat inevitable.

I suspect we will need a few variations on KindMimic -- at least 2. If you see #10158 you can see that I broke down the translations for pointers into two categories, owned_pointer and other_pointer (used for managed, indirect). This is because some properties only matter if they are owned, and others matter if they are reachable. I guess KindMimic corresponds to other_pointer (or should).

Also -- can we not use the name kind? I've been trying to move us to the term "builtin bounds" because the term "kind" has a pre-existing usage (higher-kinded types).

[nikomatsakis]

Thinking more about this-- maybe the right answer is just to make *T transmite the "reachable" constraints for T