extern mod => extern crate

This was previously implemented, and it just needed a snapshot to go through

src/libserialize/lib.rs

@@ -24,7 +24,7 @@ Core encoding and decoding interfaces.
 
 // test harness access
 #[cfg(test)]
-extern mod extra;
+extern crate extra;
 
 pub use self::serialize::{Decoder, Encoder, Decodable, Encodable,
     DecoderHelpers, EncoderHelpers};