Eliminated a signal by using a simpler trait method that returns whether VBus power is available. Also includes a UsbSupply that can be signalled for use with the nRF softdevice. Includes the requirement for waiting for power to become available.
I've renamed the channel module for the MPMC as mpmc. There was a previous debate about this, but I feel that the strategy here avoids importing `channel::channel`. The change leaves `signal::Signal`, but I think that's ok. It is all a bit subjective of course. The bottom line for me is that I really like the term mpmc - it means something to me and aligns with broader naming e.g. in Tokio.
