std: Modernize the local_data api

This commit brings the local_data api up to modern rust standards with a few key
improvements:

* The `pop` and `set` methods have been combined into one method, `replace`

* The `get_mut` method has been removed. All interior mutability should be done
  through `RefCell`.

* All functionality is now exposed as a method on the keys themselves. Instead
  of importing std::local_data, you now use "key.replace()" and "key.get()".

* All closures have been removed in favor of RAII functionality. This means that
  get() and get_mut() no long require closures, but rather return
  Option<SmartPointer> where the smart pointer takes care of relinquishing the
  borrow and also implements the necessary Deref traits

* The modify() function was removed to cut the local_data interface down to its
  bare essentials (similarly to how RefCell removed set/get).

[breaking-change]

src/libstd/macros.rs

@@ -289,12 +289,10 @@ macro_rules! println(
 /// # Example
 ///
 /// ```
-/// use std::local_data;
-///
 /// local_data_key!(my_integer: int)
 ///
-/// local_data::set(my_integer, 2);
-/// local_data::get(my_integer, |val| println!("{}", val.map(|i| *i)));
+/// my_integer.replace(Some(2));
+/// println!("{}", my_integer.get().map(|a| *a));
 /// ```
 #[macro_export]
 macro_rules! local_data_key(