Fix: Utf16 handling moved from std into unicode crate

Following a rust change.

serde2/src/lib.rs

@@ -1,5 +1,7 @@
 #![feature(macro_rules)]
 
+extern crate unicode;
+
 pub use ser::{Serialize, Serializer};
 //pub use ser::{Visitor, VisitorState};
 //pub use ser::GatherTokens;