Remove parameters from (future) .return(…) method

[parzhitsky]

Protocol-compliant implementation assumes .return(…) method having a parameter, that's basically passed through as the return value of the method's call. Since xrange is a number iterator, it is designed to return "nothing" (i.e., undefined) after the iterated range is finished (the current NaN is temporary as per parzh/xrange#50), which means that the returned value of .return(…) is always an implicit undefined, and it doesn't depend on any inputs.

[parzhitsky]

Blocked by #13