This change is specifically intended to avoid a pitfall in MicroPython and will likely have no effect in C++.
When using the REPL in MicroPython it's possible to set up an I2C instance on two pins - ie: 20, 21 - and then subsequently realise these are the wrong pins for your board.
Before this change, these pins would be left hanging even if you created a new I2C instance with new pins - ie: 4, 5 - this would lead to communications failures where they really shouldn't happen. Confusing!
Removes all driver-specific SDA/SCL pin definitions and defaults.
Pin type is "uint" everywhere, but "PIN_UNUSED" is *int*_max for MicroPython compat. That's still a lot of pins!
Adds baudrate to the I2C class, and allows a driver (like Trackball) to check the baudrate is supported
This change adds a common I2C class, gathering various I2C functions into a single point of responsibility.
It's necessary for correctly managing the I2C bus pins and state across multiple devices.
Phil Howard