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)
The call to `black_box` is far more expensive than a TLS access, and turning off optimizations won't yield useful numbers without a lot of effort put into crafting a real benchmark. The cost of accessing TLS in the executable is comparable to the cost of accessing a global variable or data on the stack.