Add annotations for structs, enums and variants.

[josefrcm]

This is required by some libraries, for example:

• Serde enum representations (https://serde.rs/enum-representations.html)
• Askama templates (https://djc.github.io/askama/creating_templates.html)
• postgres-types (https://docs.rs/postgres-types/0.2.0/postgres_types/)

[Ben-PH]

also needed for crate modular-bitfield