Helper derive macros for enums

• AllVariantsSerdeRenames: creates a method fn all_variants_serde_renames() -> Vec<String>, returning all variants serde rename attributes data.
• VariantName: crates a method fn variant_name(&self) -> &str, returning the serde rename attribute data.

NOTE!

Both macros work only if there is #[serde(rename=...)] attribute applied to all of the enum variants