Low Power Shared API

As per ongoing discussion with .org about libraries, here is a proposal for MCU (and companion MCU) standby API

https://github.com/rocketscream/Low-Power is a good starting point, AVR API is too granular so we should avoid it. But the SAMD API is quite good.

Function calls would simply be
- `idle()`
- `sleep()`
- `deepSleep()` -> might call `sleep()` if there is only one sleep mode

which in modern MCU is like WFI with peripherals enabled and WFI with peripherals disabled.
STM32 processors also have a mode called WFE which I would avoid exposing. Other MCU (nRF chips for example) might have multiple sleep levels; I'd select two/three of them to be exposed.

To register a wakeup source I propose overloading `attachInterrupt` API with a couple of versions:
- `attachInterrupt(pin, function, MODE, wakeup)` would become the main function;
- `attachInterrupt(pin, function, MODE, false)` will be called for classic API
- `attachInterruptWakeup(pin, function, MODE)` will call `attachInterrupt(pin, function, MODE, true)`

If function is NULL, wakeup-enable bit should be set and no function called

if `pin` in `NOT_A_PIN`, extra code can handle the additional wakeup sources.
I suggest using macros like `RTC_WAKEUP` or `ADC_WAKEUP` with high values (0xFF, 0FE etc)

@cmaglie @sandeepmistry @tigoe any thoughts about that?

Start with the proposed idle(), sleep(), and deepSleep() entry points and the attachInterrupt overloads described in the issue. Review the linked Low-Power library and SAMD API as references, then determine the supported MCU behavior and wakeup-source design. Done means the API proposal has a decided scope and an agreed implementation path.

Written by the indexing model from the issue text.