message: Mark the unsafe functions in OwnedFrozenValue as unsafe
  Summary: Previously they just had a big warning that they are likely to gain unsafe in future. Now we have safe alternatives, we're in a position to do this without impacting too much.
diff: @@ -81,7 +81,8 @@ impl OwnedFrozenValue {
  /// see the [`frozen_heap`](crate::environment::Module::frozen_heap) function.
  /// If you don't care about the resulting lifetime the [`value`](OwnedFrozenValue::value) method is easier.
  pub fn owned_value<'v>(&self, heap: &'v FrozenHeap) -> Value<'v> {
  - self.owned_frozen_value(heap).to_value()
  + // Safe because we convert it to a value which is tied to the owning heap
  + unsafe { self.owned_frozen_value(heap).to_value() }
  }
  /// Operate on the [`FrozenValue`] stored inside.
  @@ -94,26 +95,20 @@ impl OwnedFrozenValue {
  }
  }
  - /// Be VERY careful, potential segfault! See the warnings on `OwnedFrozenValue`.
  - /// This function is highly likely to gain an `unsafe` in the future.
  - ///
  /// Obtain direct access to the [`FrozenValue`] that lives inside. If you drop all
  /// references to the [`FrozenHeap`] keeping it alive, any code using the [`FrozenValue`]
  /// is likely to segfault. If possible use [`value`](OwnedFrozenValue::value) or
  /// [`owned_frozen_value`](OwnedFrozenValue::owned_frozen_value).
  - pub fn unchecked_frozen_value(&self) -> FrozenValue {
  + pub unsafe fn unchecked_frozen_value(&self) -> FrozenValue {
  self.value
  }
  - /// Be VERY careful, potential segfault! See the warnings on `OwnedFrozenValue`.
  - /// This function is highly likely to gain an `unsafe` in the future.
  - ///
  /// Extract a [`FrozenValue`] by passing the [`FrozenHeap`] which will keep it alive.
  /// Provided the argument heap does indeed stay alive for the lifetime of the result,
  /// all will be fine. Unsafe if you pass the wrong heap, or don't keep the heap alive
  /// long enough. Where possible, use [`value`](OwnedFrozenValue::value) or
  /// [`owned_value`](OwnedFrozenValue::owned_value).
  - pub fn owned_frozen_value(&self, heap: &FrozenHeap) -> FrozenValue {
  + pub unsafe fn owned_frozen_value(&self, heap: &FrozenHeap) -> FrozenValue {
  heap.add_reference(&self.owner);
  self.value
  }
  