Implement deserializer for map mode and collection fields

serde/src/export.rs

@@ -13,6 +13,7 @@ pub use lib::fmt::{self, Formatter};
 pub use lib::marker::PhantomData;
 pub use lib::option::Option::{self, None, Some};
 pub use lib::result::Result::{self, Err, Ok};
+pub use lib::iter::once;
 
 pub use self::string::from_utf8_lossy;