Before we'd generate all pins Px0..Px15 for each GPIOx port. This changes
codegen to only generate singletons for actually-existing pins.
(AFs were already previously filtered, so these non-existing pins were already mostly useless)
- usart works
- dac works
- rng gets stuck on while loop
- usb_serial works, but cannot test due to lack of user usb port
- adc needs work and does not work yet
