separate convesions from the main source to allow test builds
extra unit to verify that both duration plain input and
spec work as intended

also fix wdt triggered when unknown symbols are encountered at the end
of duration string (e.g. 10000y)

solves edge-case when we move our storage nearby a previously erased kv,
causing it to be treated as a key or a value with length 255 (... 0x00 0xff)

also stop set() from erasing data when it is not possible to store it in
the first place

additional test case similar to permutations one, make sure keys don't
disappear while we write to the storage a lot of times and possibly
introduce the two cases 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

Resolve the issue with the UnixHostDuino not really being compatible
with the esp8266 Core String (...and the rest of the Core, as well)

Port the CMakeLists.txt from the rpnlib and update it use FetchContent
instead of either manually fetching dependencies or using PIO artifacts
Caching is *expected* to work, but might need slight adjustments