[refactor] #132: refactor codegen for enum

[QuentinI]

Finally no more metaclass voodoo in codegen, also (probably) more pythonic types for rust enums.

Next steps would be to refactor codegen on the rust side, then probably move as much of codegen as possible from python to rust for better IDE/typechecking experience for end-users.

Closes #132, also at least partially addresses #134