auto merge of #7751 : alexcrichton/rust/finish-tls, r=pcwalton

auto merge of #7751 : alexcrichton/rust/finish-tls, r=pcwalton

This changes the interface to `get`, and it also changes the keys to be static slices instead of static functions.

This allows the removal of the `unsafe` interface because while functions can monomorphize from different types to the same actual function, static slices cannot do this.

From at least what I can tell, we don't need to worry about LLVM coalescing these addresses. If we ever use the `unnamed_addr` it looks like there's cause for worry, but there doesn't appear to be any coalescing atm.