Only include sexp module if compare feature is enabled.

src/context/context.rs

@@ -28,6 +28,7 @@ pub(crate) enum ContextElement<'r, 's> {
     ConsumeTrailingWhitespace(bool),
 
     /// This is just here to use the 's lifetime until I'm sure we can eliminate it from ContextElement.
+    #[allow(dead_code)]
     Placeholder(PhantomData<&'s str>),
 }