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'm actually not super familiar with the semantics of C++11 destructors with respect to `thread_local`, do you know if there's some documentation that you can point me at? The sample implementation actually does this already where it favors the oddly-named destructor registration functions over an OS-based implementation, but the OS-based implementation is provided as a fallback.