RPi I/O Bus Instantiation

[CaiB]

Currently, users are expected to make and manage a single instance of I/O buses (UART, SPI, and I2C) on the RPi. We should change this to be the same as on the BBB, where instances are provided by the I/O system.

Currently, if the user makes two instances, everything works, but thread safety is no longer guaranteed, which can have confiusing effects. That, and the fact that the two platforms use different instantiation patterns is simply confusing.

This will be a breaking change.