Implement Complex#<=> so that it is usable as an argument when
calling <=> on objects of other classes (since #coerce will coerce
such numbers to Complex). If the complex number has a zero imaginary
part, and the other argument is a real number (or complex number with
zero imaginary part), return -1, 0, or 1. Otherwise, return nil,
indicating the objects are not comparable.
Fixes [Bug #15857]
