code: public static <T> Ix<T> repeatCallable(Callable<T> callable) {
          return new IxRepeatCallable<T>(nullCheck(callable, "callable is null"));
      }
docs: Repeatedly calls the given callable indefinitely and
  emits the returned value.
  <p>
  The result's iterator() doesn't support remove().
  @param <T> the value type
  @param callable the callable to call
  @return the new Ix instance
  @since 1.0