message: Do not create maps in MutableNames
  Summary: Minor code cleanup.
diff: @@ -96,7 +96,7 @@ impl MutableNames {
  self.0.borrow_mut().remove(name);
  }
  - pub fn all_names(&self) -> IndexMap<String, ModuleSlotId> {
  + pub(crate) fn all_names(&self) -> Vec<(String, ModuleSlotId)> {
  self.0
  .borrow()
  .iter()
  @@ -104,7 +104,7 @@ impl MutableNames {
  .collect()
  }
  - pub(crate) fn all_names_and_visibilities(&self) -> IndexMap<String, Visibility> {
  + pub(crate) fn all_names_and_visibilities(&self) -> Vec<(String, Visibility)> {
  self.0
  .borrow()
  .iter()
  