Added box_syntax feature gate; added to std and rustc crates for bootstrap.

To avoid using the feauture, change uses of `box <expr>` to
`Box::new(<expr>)` alternative, as noted by the feature gate message.

(Note that box patterns have no analogous trivial replacement, at
least not in general; you need to revise the code to do a partial
match, deref, and then the rest of the match.)

[breaking-change]

src/liballoc/lib.rs

@@ -66,6 +66,7 @@
 #![no_std]
 #![allow(unknown_features)]
 #![feature(lang_items, unsafe_destructor)]
+#![feature(box_syntax)]
 
 #[macro_use]
 extern crate core;