message: Don't test for mutable in thaw_on_write
  Summary: We only call thaw_on_write from two specific places, both of which we know are mutable, so no need to do this test here.
diff: @@ -242,11 +242,7 @@ impl Heap {
  }
  pub(crate) fn alloc_thaw_on_write<'v>(&'v self, x: FrozenValue) -> Value<'v> {
  - if x.get_ref().naturally_mutable() {
  self.alloc_raw(ValueMem::ThawOnWrite(ThawableCell::new(x)))
  - } else {
  - Value::new_frozen(x)
  - }
  }
  pub fn alloc_mutable<'v>(&'v self, x: impl MutableValue<'v>) -> Value<'v> {
  