Mostly optimizing TLS accesses to bring local heap allocation performance
closer to that of oldsched. It's not completely at parity but removing the
branches involved in supporting oldsched and optimizing pthread_get/setspecific
to instead use our dedicated TCB slot will probably make up for it.

Document `pub` requirement for `new_without_default` lint

fixes rust-lang#8415

Also adds some UI tests that ensure that `pub` is required on both the struct _and_ the field. The only thing I'm not sure about is that the lint actually [checks](https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/new_without_default.rs#L102) if `new` is _reachable_, not _public_. To the best of my understanding, both the struct and the method need to be public for the method to be reachable for external crates (I certainly didn't manage to craft a counterexample).

changelog: Document `pub` requirement for ``[`new_without_default`]`` lint.