message: Make sig registry private
diff: @@ -16,10 +16,8 @@ use std::sync::Arc;
  pub struct Instance {
  backing: LocalBacking,
  import_backing: ImportBacking,
  -
  + sig_registry: SigRegistry,
  pub module: Arc<Module>,
  -
  - pub sig_registry: SigRegistry,
  }
  impl Instance {
  