Set allow(unstable) in crates that use unstable features

Lets them build with the -dev, -nightly, or snapshot compiler

src/liballoc/lib.rs

@@ -66,6 +66,7 @@
 
 #![no_std]
 #![allow(unknown_features)]
+#![allow(unstable)]
 #![feature(lang_items, unsafe_destructor)]
 #![feature(box_syntax)]
 #![feature(optin_builtin_traits)]