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)
Note that the sample implementation does indeed use the destructor registration functions, and it does actually have a statically initialized variant, I just felt that it was confusing to expose so many variants of TLS. I'll benchmark this though so we can get a good handle on how big of a hit this is.