Fix build by making the registry guarded by an ArcMutex.

src/intermediate/footnote_reference.rs

@@ -1,4 +1,5 @@
 use super::macros::intermediate;
+use super::registry::get_footnote_reference_id;
 use super::registry::Registry;
 use crate::error::CustomError;
 
@@ -9,9 +10,8 @@ pub(crate) struct IFootnoteReference {
 }
 
 intermediate!(IFootnoteReference, FootnoteReference, original, registry, {
-    let footnote_id = registry
-        .get_footnote_reference_id(original.label, &original.definition)
-        .await?;
+    let footnote_id =
+        get_footnote_reference_id(registry, original.label, &original.definition).await?;
     Ok(IFootnoteReference {
         footnote_id,
         duplicate_offset: 0, // TODO