zvariant-2.7.0

• Upgrade nix and rand dependencies.
• Assert auto trait impls for pub items.
• impl PartialEq<str> for {Signature, Str, ObjectPath}.
• Add Value and OwnedValue derive macros
• Add Structure::into_fields().
• Impl a bunch of missing OwnedValue conversions.
• impl From<{OwnedSignature,OwnedObjectPath}> for Value<'static>
• Implement conversion from Str to Value.
• Add some tuple conversions.
• impl From<OwnedValue> for Value<'static>.
• Fix doc reference warnings.
• Fix build with gvariant feature disabled.