Duplicate definitions of type vars.

[dlech]

The code generator currently emits TypeVars for each class that uses them. However most classes reuse the same common names like T, K and V. Mypy complains about this with Cannot redefine "T" as a type variable mypy(error) and Invalid assignment target mypy(error).

To fix this, we would probably need to make a separate pass that iterates all of the classes in a module and collects the generic type arguments and only emits unique ones at the beginning of the module.