* stateless callbacks for mqttRegister. no module uses the lambda / std
  func with captures or otherwise, so it is kind of pointless to have it
* make sure we know topic and payload length at all invocations and
  not constantly trying to re-parse the same string over and over again
* clean-up api related to string parsing and make sure we allow
  stringview as input
* clean-up id / unsigned number parsing to work with views instead of
  using a generic strto{,u}l; sometimes this works, sometimes doesn't.
  as noticed previously with IR implementation, it *will* parse until
  the '\0' is found in the input and we can't interpret parts of the
  string without doing a copy for the strtoul
* fixing additional bugs caught in ifan, leds, sensors and lights that
  were causing build failures. plus, more range-based parsing code for
  the same reason as described above

* globally accessible system timer class; help out with our internal
  scheduling by always using strongly typed duration and dynamic
  time adjustment for durations longer than system limits
  (see `os_timer_t` documentation)
* our own class for scheduled callbacks which are either choosing a
  simple function pointer or std function wrapper, depending on the type
  passed into the constructor. specifically for scheduled functions,
  this allows us to filter globally scheduled functions and push them to
  the front of the queue when necessary to ensure certain order of calls
* replace Ticker instances with SystemTimer
* allow types {h, cpp} in unit tests