Set allow(unstable) in crates that use unstable features

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

src/librand/lib.rs

@@ -23,6 +23,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 #![allow(unknown_features)] #![feature(int_uint)]
+#![allow(unstable)]
 #![no_std]
 #![unstable]
 #![staged_api]