Add enum support

[pbedn]

Summary

• add EnumMember and EnumDef nodes
• parse enum definitions
• type check enums and allow using enum members in expressions
• generate C enums and print names
• provide stdlib enum stub and example
• test enum parsing, typing, codegen and runtime

Testing

• pytest -q

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https://chatgpt.com/codex/tasks/task_e_687e24b8ea388321832be5fcf97fa041