This contains the pre-1.0 breaking API changes and USB serial re-work.

This should improve serial issues after first physical connection + flash and extra serial from previous programs leaking into subsequent ones.

This is a preview version for testing on a review branch, we won't merge this directly.

I've also changed the default logger to a console logger as it's annoying to get less log output without the micro:bit theme - particularly important for testing the connection library and getting its logs.