Add enumeration types?

Enum types are quite useful in many specifications, add support for these to c2po. 

For example:
```
ENUM 
    RequestState: {Waiting: 0, Granted: 1, Rejected: 2};

INPUT
    state: RequestState;

FTSPEC
    (state == Waiting) U[0,5] (state == Granted || state == Rejected);
```

The user will have to consistently map the enum values in the traces though.