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]

src/librustdoc/markdown.rs

@@ -55,7 +55,7 @@ pub fn render(input: &str, mut output: Path, matches: &getopts::Matches,
     let input_str = load_or_return!(input, 1, 2);
     let playground = matches.opt_str("markdown-playground-url");
     if playground.is_some() {
-        markdown::playground_krate.replace(Some(None));
+        markdown::PLAYGROUND_KRATE.with(|s| { *s.borrow_mut() = None; });
     }
     let playground = playground.unwrap_or("".to_string());