Factor helper code out of generated code

Nobody likes coding inside a string literal!
diff --git a/src/lib.rs b/src/lib.rs
index 1fa2eed..a8c4a39 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -113,6 +113,9 @@
 
     #[cfg(feature = "fold")]
     pub mod fold;
+
+    #[path = "../gen_helper.rs"]
+    mod helper;
 }
 pub use gen::*;