PD3.0 spec requires concurrent control of CC resistors for collision avoidance.
Needed to introduce some "ref counting" (its just a bool) for drop code.
`select(rx.receive(), tx.transmit()` had subtle interrupt enable race conditions.
Combine receiver and transmitter into one new `PdPhy` struct to disallow the
problematic pattern.
Scanning through the USB PD 2.0 specification there is no need to have RX and TX
running concurrently (after all the USB PD communication is half-duplex).
* Improved interrupt handling: Clear flags in ISR, check state change in future
* Disable pull-up/pull-down resistors and voltage monitor on drop
* nightly rustfmt
