message: Remove a clippy warning that no longer fires
  Summary: The type no longer returns mut, so can delete this warning. I audited the rest of Starlark and there were no other dead warnings.
diff: @@ -111,7 +111,6 @@ impl Arena {
  }
  }
  - #[allow(clippy::mut_from_ref)] // This is fine for arenas
  pub(crate) fn alloc<'v, 'v2: 'v, T: AValue<'v2>>(&'v self, x: T) -> &'v AValuePtr {
  let p = self.alloc_empty::<T>();
  unsafe {
  