message: No need to borrow arenas twice
  Summary: They used to be a Ref, now they are just references, so can use them directly.
diff: @@ -413,8 +413,7 @@ impl Heap {
  }
  fn alloc_raw<'v, 'v2: 'v2>(&'v self, x: impl AValue<'v2, ExtraElem = ()>) -> Value<'v> {
  - let arena_ref = self.arena.borrow();
  - let arena = &*arena_ref;
  + let arena = self.arena.borrow();
  let v: &AValueRepr<_> = arena.alloc(x);
  // We have an arena inside a RefCell which stores ValueMem<'v>
  @@ -438,8 +437,7 @@ impl Heap {
  len: usize,
  init: impl FnOnce(*mut u8),
  ) -> StringValue<'v> {
  - let arena_ref = self.arena.borrow();
  - let arena = &*arena_ref;
  + let arena = self.arena.borrow();
  let (v, extra) = arena.alloc_extra_non_drop::<_>(starlark_str(len));
  init(extra.as_mut_ptr() as *mut u8);
  