Create an new interval with `step` multiplied by `step_factor`.

This is typically applied to an interval with implicit `step` of one,
e.g., in `2..10:2` will create "{2,4,6,8,10}".

pre
  debug: ((step_factor.sign = 0): through=from)  # step cannot be zero unless from=through
  safety: (step *? step_factor).exists