RFC: Add a thread local storage module, std::tls

Introduce a new thread local storage module to the standard library, `std::tls`,
providing:

* Scoped TLS, a non-owning variant of TLS for any value.
* Owning TLS, an owning, dynamically initialized, dynamically destructed
  variant, similar to `std::local_data` today.

[Rendered](https://github.com/alexcrichton/rfcs/blob/tls-overhaul/text/0000-tls-overhaul.md)
I think it's wrong for the `get` function to return `Option`. This isn't a runtime error for the program to handle, it's a bug. The *only* sane thing to do with it is to `unwrap` it because a correct program will not have these cycles.