DSTify Str + impl Str for &S where S: Str
Require <T: Send> for AtomicOption

Fixes #19247.
std: Add a new top-level thread_local module

This commit removes the `std::local_data` module in favor of a new
`std::thread_local` module providing thread local storage. The module provides
two variants of TLS: one which owns its contents and one which is based on
scoped references. Each implementation has pros and cons listed in the
documentation.

Both flavors have accessors through a function called `with` which yield a
reference to a closure provided. Both flavors also panic if a reference cannot
be yielded and provide a function to test whether an access would panic or not.
This is an implementation of [RFC 461][rfc] and full details can be found in
that RFC.

This is a breaking change due to the removal of the `std::local_data` module.
All users can migrate to the new thread local system like so:

    thread_local!(static FOO: Rc<RefCell<Option<T>>> = Rc::new(RefCell::new(None)))

The old `local_data` module inherently contained the `Rc<RefCell<Option<T>>>` as
an implementation detail which must now be explicitly stated by users.

[rfc]: https://github.com/rust-lang/rfcs/pull/461
[breaking-change]
auto merge of #19094 : alexcrichton/rust/rm-std-local-data, r=aturon

This commit removes the `std::local_data` module in favor of a new `std::thread_local`
module providing thread local storage. The module provides two variants of TLS:
one which owns its contents and one which is based on scoped references. Each
implementation has pros and cons listed in the documentation.

Both flavors have accessors through a function called `with` which yield a
reference to a closure provided. Both flavors also panic if a reference cannot
be yielded and provide a function to test whether an access would panic or not.
This is an implementation of [RFC 461][rfc] and full details can be found in
that RFC.

This is a breaking change due to the removal of the `std::local_data` module.
All users can migrate to the new tls system like so:

    thread_local!(static FOO: Rc<RefCell<Option<T>>> = Rc::new(RefCell::new(None)))

The old `local_data` module inherently contained the `Rc<RefCell<Option<T>>>` as
an implementation detail which must now be explicitly stated by users.

[rfc]: https://github.com/rust-lang/rfcs/pull/461
[breaking-change]
auto merge of #19248 : japaric/rust/str, r=alexcrichton

Just like we do with AsSlice

This comes in handy when dealing with iterator-centric APIs (`IntoIterator`!) and you want to receive an `Iterator<S> where S: Str` argument. Without this PR, e.g. you can't receive `&["a", "b"].iter()` instead you'll have to type `&["a", "b"].iter().map(|&x| x)` (A similar thing happens with `&[String]`).

r? @aturon 

Full disclaimer: I haven't run `make`/`make check` yet (All my cores are busy)
auto merge of #19250 : kmcallister/rust/atomicoption, r=alexcrichton

Fixes #19247.