zvariant-2.8.0

• Add Optional<T> type. See API docs for the need and use for this type.
• Deprecate unneeded Infallible variant.
• Avoid some unneeded allocations.
• Add Str::into_owned method.
• Impl Default for Str.
• Implement Type for std::borrow::Cow<'_, T: Type>.
• Mark Error as non-exhaustive.