Factor helper code out of generated code

Nobody likes coding inside a string literal!
diff --git a/src/gen/fold.rs b/src/gen/fold.rs
index d8fbd91..2d6a92d 100644
--- a/src/gen/fold.rs
+++ b/src/gen/fold.rs
@@ -10,30 +10,8 @@
 #![cfg_attr(feature = "cargo-clippy", allow(needless_pass_by_value))]
 
 use *;
-use delimited::Delimited;
 use proc_macro2::Span;
-
-trait FoldHelper {
-    type Item;
-    fn lift<F>(self, f: F) -> Self where F: FnMut(Self::Item) -> Self::Item;
-}
-
-impl<T> FoldHelper for Vec<T> {
-    type Item = T;
-    fn lift<F>(self, f: F) -> Self where F: FnMut(Self::Item) -> Self::Item {
-        self.into_iter().map(f).collect()
-    }
-}
-
-impl<T, U> FoldHelper for Delimited<T, U> {
-    type Item = T;
-    fn lift<F>(self, mut f: F) -> Self where F: FnMut(Self::Item) -> Self::Item {
-        self.into_iter().map(|elem| {
-            let (t, u) = elem.into_tuple();
-            (f(t), u)
-        }).collect::<Vec<(T, Option<U>)>>().into()
-    }
-}
+use gen::helper::fold::*;
 
 
 #[cfg(feature = "full")]