restore len(alarm.sleep_memory) and bool(alarm.sleep_memory)

[dhalbert]

Fixes #3956.

This was originally copied over from microcontroller.nvm, and then after a discussion with @tannewt, was deleted. I didn't mean to delete len() . And on reflection, I think bool() can stay: it's small and cheap, and there's a off-chance that an implementation might provide a zero-length alarm.sleep_memory(say, for instance, its size was dynamically chosen) and that needs to have proper falsiness.

[anecdata]

Adafruit CircuitPython 6.1.0-rc.0-13-ga589c2630 on 2021-01-09; Saola 1 w/Wrover with ESP32S2
>>> import alarm
>>> len(alarm.sleep_memory)
4096

Thanks, Dan! I looked at it and tried a few things, but I was way off.

[tannewt]

Looks good! Thanks!