message: Don't allow users to allocate thaw-on-write things
  Summary: It's an internal optimisation that isn't widely applicable, so keep it internal.
diff: @@ -241,7 +241,7 @@ impl Heap {
  self.alloc_raw(ValueMem::Immutable(box x))
  }
  - pub fn alloc_thaw_on_write<'v>(&'v self, x: FrozenValue) -> Value<'v> {
  + 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 {
  