add a few blanket future impls to std

src/liballoc/lib.rs

@@ -80,6 +80,7 @@
 #![cfg_attr(test, feature(rand, test))]
 #![feature(allocator_api)]
 #![feature(allow_internal_unstable)]
+#![feature(arbitrary_self_types)]
 #![feature(ascii_ctype)]
 #![feature(box_into_raw_non_null)]
 #![feature(box_patterns)]