message: Make the RegistryImpl() constructor public.
  The target is still build visibility restricted, and hence should not be used by anyone.
diff: @@ -49,6 +49,10 @@ class RegistryImpl {
  return *instance;
  }
  + RegistryImpl() = default;
  + RegistryImpl(const RegistryImpl&) = delete;
  + RegistryImpl& operator=(const RegistryImpl&) = delete;
  +
  template <class P>
  crypto::tink::util::StatusOr<const Catalogue<P>*> get_catalogue(
  const std::string& catalogue_name) const ABSL_LOCKS_EXCLUDED(maps_mutex_);
  @@ -288,10 +292,6 @@ class RegistryImpl {
  const std::string type_id_name;
  };
  - RegistryImpl() = default;
  - RegistryImpl(const RegistryImpl&) = delete;
  - RegistryImpl& operator=(const RegistryImpl&) = delete;
  -
  template <class P>
  crypto::tink::util::StatusOr<const PrimitiveWrapper<P>*> get_wrapper() const
  ABSL_LOCKS_EXCLUDED(maps_mutex_);
  