cache attributes of items from foreign crates

this avoids parsing item attributes on each call to `item_attrs`, which takes
off 33% (!) of translation time and 50% (!) of trans-item collection time.

src/liballoc/lib.rs

@@ -87,6 +87,7 @@
 #![feature(needs_allocator)]
 #![feature(optin_builtin_traits)]
 #![feature(placement_in_syntax)]
+#![cfg_attr(stage0, feature(pub_restricted))]
 #![feature(shared)]
 #![feature(staged_api)]
 #![feature(unboxed_closures)]