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)
Do you know of documentation for the C++11 destructor semantics in thread_local? I'll reiterate that we *do* use the destructor registration functions that it uses when available, I'd like to copy the semantics to the fallback implementation (OS TLS) as much as possible, however.