fix for #622 - support start in enumerate

limited to non-keyword (positional).
generates a TypingError if the start value is not an integral type.
Yes, and in fact I think that in many cases we should do more type checking during type inference to provide more meaningful error diagnosis, as in many cases we just let the code inference to fail on an assumption and get weird errors (like an error on not being able to unpack a tuple when the actual error is that the wrong number of arguments was used.