This is required for MicroPython error text compression which is
going to be enabled by default for RP2 builds.
We have a lot of error messages, so the potential flash gain here
is worth the effort of making this work.
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