Enums as tagged unions

Rust enums are real sum types: each variant can carry different data. This is the closest Rust analogue to TypeScript discriminated unions, but the compiler enforces exhaustiveness. Planned exercises: 1. choose enum syntax. 2. recognise tuple-like and struct-like variants. 3. choose an enum over string tags. 4. fill a variant constructor. 5. fill an enum type annotation. 6. write a small enum. 7. write a value of a data-carrying variant. 8. translate a TS discriminated union. 9. model a small state machine with enum variants.