Use proc-macro2's Span instead of our own
diff --git a/src/gen/fold.rs b/src/gen/fold.rs
index 9fc80a7..0d944e6 100644
--- a/src/gen/fold.rs
+++ b/src/gen/fold.rs
@@ -10,6 +10,7 @@
 
 use *;
 use delimited::Delimited;
+use proc_macro2::Span;
 
 trait FoldHelper {
     type Item;