Set allow(unstable) in crates that use unstable features

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

src/librustdoc/lib.rs

@@ -20,6 +20,7 @@
 #![feature(slicing_syntax)]
 #![feature(box_syntax)]
 #![allow(unknown_features)] #![feature(int_uint)]
+#![allow(unstable)]
 
 extern crate arena;
 extern crate getopts;