Fix build by making the registry guarded by an ArcMutex.

src/intermediate/mod.rs

@@ -124,3 +124,6 @@ pub(crate) use timestamp::ITimestamp;
 pub(crate) use underline::IUnderline;
 pub(crate) use verbatim::IVerbatim;
 pub(crate) use verse_block::IVerseBlock;
+
+pub(crate) type RefRegistry<'orig, 'parse> =
+    std::sync::Arc<std::sync::Mutex<registry::Registry<'orig, 'parse>>>;