v0.2.0 easier GPIO control

[x] five-state GPIO can be now (more) intuitively controlled by gpio_out(), gpio_pull() and gpio_highz() functions
[x] fixed a little bug that prevented the use of GPIO for stepper endstop
[x] RP2040-Zero board tested OK - troubles with oscillator start-up just required compilation against new (2023+) SDK

Full Changelog: v0.1.0...v0.2.0

v0.1.0 First release for real world deployment

• Technical issues seem to be resolved.
• Communication at the USB 1.0 maximum around 1 MB/s proven both on Linux (5.4) and Windows (10), even with a tight busy loop in the user script. This was fairly nontrivial to achieve.
• Multiprocessing fork bomb is no more trouble even if user script does not check for __name__=='__main__'.
• ADC, GPIO, PWM and stepper motor control seems to work, will be extensively tested on real tasks.
• Binary firmwares only available in public releases - or upon git pull and own recompilation.
• Digital protocols and many other features are to be implemented. API will change.