extern mod => extern crate

This was previously implemented, and it just needed a snapshot to go through

src/test/bench/task-perf-alloc-unwind.rs

@@ -10,8 +10,8 @@
 
 #[feature(managed_boxes)];
 
-extern mod extra;
-extern mod collections;
+extern crate extra;
+extern crate collections;
 
 use collections::list::{List, Cons, Nil};
 use extra::time::precise_time_s;