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)
Using the C++11 destructor support would be much more robust. It doesn't have weird limitations like `PTHREAD_DESTRUCTOR_ITERATIONS`, and a fallback implementation when the platform doesn't properly support C++11 (nearly all do) just needs to be safe rather than *perfect*.