provide a new efficient implementation of thread-local storage

[thestinger]

It should be a thin layer on top of #[thread_local] where supported. It can provide an unsafe low-level API and then implement Cell and RefCell semantics on top of that for the safe API.

[thestinger]

Blocked on #18003 for the fallback path on platforms without a working #[thread_local] and for destructor support on platforms without the building blocks used by C++11.

[alexcrichton]

Landed in #19094