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.
Remove an outdated drawback
Tweak the API of owning TLS
Merge branch 'tls-overhaul' of https://github.com/alexcrichton/rfcs into alexcrichton-tls-overhaul
RFC 461 is TLS overhaul