A `from..through` interval with a given `step`.

This is a `Set T` and a `Sequence T` created by repeatedly adding `step` to `from`.
Stops before passing `through` (exceeding it for `step > 0`, falling below for `step < 0`).

Empty if `step` points away from `through`:
interval 1 2 -1 # empty: steps down, but through is up
interval 2 1 1 # empty: steps up, but through is down


A zero `step` is only allowed when `from = through`; such an interval
contains the single element `from`:
interval 5 5 0 # 5

If `through` is `nil`, the interval is unbounded and limited only by type `T` range.

This is not an interval in the mathematical sense, but rather a representation
of an arithmetic progression.

pre
  debug: ((step.sign = 0): through = from)