Fix build by making the registry guarded by an ArcMutex.

src/intermediate/section.rs

@@ -12,7 +12,7 @@ intermediate!(ISection, Section, original, registry, {
     let children = {
         let mut ret = Vec::new();
         for elem in original.children.iter() {
-            ret.push(IElement::new(registry, elem).await?);
+            ret.push(IElement::new(registry.clone(), elem).await?);
         }
         ret
     };