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)
Indeed! I ran across that same trick when whipping up the [sample implementation](https://github.com/alexcrichton/tls-rs/blob/master/src/os.rs#L397-L516). I definitely agree that the implementation can be improved as well!