run the given `code` periodically at time `first+p*0`, `first+p*1`, `first+p*2`,
etc. until this clock has reached the time `last`.

In case any of the instants `first+p*0`, `first+p*1`, `first+p*2`, etc. happen to
be in the past when this is called or after the previous execution of `code`,
respectively, do not run `code` for that instant.

In case `period.is_infinity`, run `code` once unless `first > last`.

return the number of times the `code` was run.

pre
  debug: period.is_infinity || period.as_duration > time.duration.ns 0

post
  debug: ((last >= first): result <= ((last - first) / period.as_duration) + 1)
  debug: ((last <  first): result = 0)