Creates a new interval with the `step` multiplied by `step_factor`.
`from` and `through` remain unchanged.

Since it returns an interval, `infix :` can be chained (factors multiply):
2..10 # {2,4,6,8,10} (implicit step 1 -> new step 2)
(2..12:2):3 # {2,8} (step becomes 1 * 2 * 3 = 6)

A negative `step_factor` reverses the direction. It usually makes the
interval empty because `from` and `through` do not change:
2..10: -2 # empty: step points away from `through`

pre
  debug: ((step_factor.sign = 0): through=from) # `step_factor` cannot be zero unless `from == through`, because a zero step would never reach `through`.
  safety: (step *? step_factor)??