Add API to advance millis() and micros()

When an AVR MCU sleeps in power-down mode, it's clock stops running. Typically time is being kept using the watchdog timer, or an asynchronous timer2, to allow waking up after a specified time. However, the `millis()` and `micros()` counters are not updated during sleep, meaning that the sketch needs to jump through hoops (manually offsetting these counters, or modifying Arduino's global variables) to make various timing work. The former only works with sketch code, not libraries that rely on both counters, and the latter is obviously very ugly and non-portable.

Ideally, there would be an API that allows modifying the millis() and micros() timers by adding (or perhaps also subtracting?) a specified time from both. Most obvious would be an int32_t specifying the number of microseconds. This allows offsetting the time by about 35 minutes in one call. Alternatively, specifying both millis and micros separately would increase the range, but also complicate the code a bit (needing actual division instead of just bitshifting, I think). In either case, I'd say that both counters are always updated at the same time and stay in sync, not being updated individually.

Inizia individuando le dichiarazioni e le implementazioni di millis() e micros(). Verifica come vengono mantenuti sincronizzati entrambi i contatori, quindi stabilisci se l’API debba aggiungere o sottrarre un offset condiviso in microsecondi di tipo int32_t oppure accettare valori separati in millisecondi e microsecondi. Il lavoro è completo quando il codice relativo a sleep può far avanzare entrambi i timer insieme senza modificare le variabili globali di Arduino.

Scritto dal modello di indicizzazione a partire dal testo della issue.