message: Add OwnedFrozenValue.unpack_str
  Summary: This is safe because we have a lifetime to the OwnedFrozenValue, so use it.
diff: @@ -82,6 +82,11 @@ impl OwnedFrozenValue {
  self.value.unpack_int()
  }
  + /// Unpack the string contained in the underlying value, or [`None`] if it is not an string.
  + pub fn unpack_str(&self) -> Option<&str> {
  + self.value.unpack_str()
  + }
  +
  /// Operate on the [`FrozenValue`] stored inside.
  /// Safe provided you don't store the [`FrozenValue`] after the closure has returned.
  pub fn map(&self, f: impl FnOnce(FrozenValue) -> FrozenValue) -> Self {
  