[Cython] Some more optimisations

[Martin C. Martin]

Stefan Behnel wrote:

> One thing I'm not sure about is how to propagate undeclared function result
> types. If we can figure out what type a function has, we can use that type.
> But the function might be declared later in the code, so we wouldn't have that
> information early enough. I think that's where two passed are required.

I think you're absolutely right, and that's the standard answer to this. 
  Is making a separate pass difficult?

Best,
Martin