zvariant_derive-3.1.0

New release to go with zvariant 3.1.0.

• New signature = "..." attribute.
  • Type derive uses its value as a hardcoded signature.
  • Value and OwnedValue assume dictionary encoding for structs if value is a{sv} or dict.
• Deprecate TypeDict in favor of Type macro w/ the new signature attribute.
• Allow use of zvariant throught the zbus re-export.
• Separate README for the subcrate.