STM32G4 ADC improvements

• [x] Fix channels
• [x] Add support for ADC conversion sequences
• [x] Add support for ADC triggers
• [x] Add an advanced adc example for Nucleo-G474RE
• [x] ~Add nonstd::span because we are not ready for C++20 yet.~
• [ ] Prescaler computation: https://github.com/modm-io/modm/blob/develop/src/modm/math/algorithm/prescaler.hpp
• [ ] Test on real hardware

@salkinium Is it ok to copy span.hpp into the modm repository or should I add span-lite as a git submodule?

@salkinium Is it ok to copy span.hpp into the modm repository or should I add span-lite as a git submodule?

I don't feel super comfortable to add non-std code that will later be added to the std, particularly not for such a small and arguably replaceable use-case. Maybe if we curated our own std lib for Cortex-M too, then at least we weren't dependent on the compiler lib.