* Add Serial and GPIO registers and implement embedded_hal traits
Use VolAddress and phantom_fields to populate the SIOCNT, RCNT, and
SIODATA8 registers. Implement embedded_hal serial traits around an empty
SioSerial struct.
Hide serial read and write traits behind a "serial" feature flag
to make embedded-hal and nb dependencies optional.
* UART echo example
Enable the serial feature for this example. Provide a pinout
diagram to assist people with wiring up a USB to UART adapter.
