Set allow(unstable) in crates that use unstable features

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

src/libfmt_macros/lib.rs

@@ -26,6 +26,7 @@
 
 #![feature(slicing_syntax)]
 #![allow(unknown_features)] #![feature(int_uint)]
+#![allow(unstable)]
 
 pub use self::Piece::*;
 pub use self::Position::*;