gpython: Implement iter()

[corona10]

TODO

• Implement callable_iterator
• Implement iter() builtin

callable_itereator should be implemented before implementing iter() builtin

[corona10]

Return a new iterator. The first parameter, callable, can be any Python callable object that can be called with no parameters; each call to it should return the next item in the iteration. When callable returns a value equal to sentinel, the iteration will be terminated.

https://docs.python.org/3/c-api/iterator.html#c.PyCallIter_New