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/librustdoc/lib.rs

@@ -28,7 +28,6 @@ extern crate time;
 extern crate log;
 extern crate libc;
 
-use std::local_data;
 use std::io;
 use std::io::{File, MemWriter};
 use std::str;
@@ -276,7 +275,7 @@ fn rust_input(cratefile: &str, matches: &getopts::Matches) -> Output {
                        &cr)
     }).unwrap();
     info!("finished with rustc");
-    local_data::set(analysiskey, analysis);
+    analysiskey.replace(Some(analysis));
 
     // Process all of the crate attributes, extracting plugin metadata along
     // with the passes which we are supposed to run.