zvariant_derive-2.7.0

New release to go with zvariant 2.7.0.

• Upgrade proc-macro-crate to 1.0.0
• Allow zvariant_derive macros to be used without prelude imports
• Fix use of unqualified zvariant path in generated code
• Add Value and OwnedValue derive macros
• Other minor and/or internal fixes and improvements.