auto merge of #15591 : aturon/rust/box-cell-stability, r=alexcrichton

This PR is the outcome of the library stabilization meeting for the
`liballoc::owned` and `libcore::cell` modules.

Aside from the stability attributes, there are a few breaking changes:

* The `owned` modules is now named `boxed`, to better represent its
  contents. (`box` was unavailable, since it's a keyword.) This will
  help avoid the misconception that `Box` plays a special role wrt
  ownership.

* The `AnyOwnExt` extension trait is renamed to `BoxAny`, and its `move`
  method is renamed to `downcast`, in both cases to improve clarity.

* The recently-added `AnySendOwnExt` extension trait is removed; it was
  not being used and is unnecessary.

[breaking-change]

src/test/compile-fail/new-box-syntax-bad.rs

@@ -14,7 +14,7 @@
 // Tests that the new `box` syntax works with unique pointers and GC pointers.
 
 use std::gc::{Gc, GC};
-use std::owned::{Box, HEAP};
+use std::boxed::{Box, HEAP};
 
 pub fn main() {
     let x: Gc<int> = box(HEAP) 2;  //~ ERROR mismatched types

src/test/run-pass/new-box-syntax.rs

@@ -14,7 +14,7 @@
 // Tests that the new `box` syntax works with unique pointers and GC pointers.
 
 use std::gc::{Gc, GC};
-use std::owned::{Box, HEAP};
+use std::boxed::{Box, HEAP};
 
 struct Structure {
     x: int,
@@ -33,4 +33,3 @@ pub fn main() {
     let c = box()(3i + 4);
     let d = box(GC)(5i + 6);
 }
-

src/test/run-pass/unit-like-struct-drop-run.rs

@@ -10,7 +10,7 @@
 
 // Make sure the destructor is run for unit-like structs.
 
-use std::owned::AnyOwnExt;
+use std::boxed::BoxAny;
 use std::task;
 
 struct Foo;
@@ -26,6 +26,6 @@ pub fn main() {
         let _b = Foo;
     });
 
-    let s = x.unwrap_err().move::<&'static str>().unwrap();
+    let s = x.unwrap_err().downcast::<&'static str>().unwrap();
     assert_eq!(s.as_slice(), "This failure should happen.");
 }