Split HW traits to support different configurations

[MorganR]

This makes writing the drivers a little annoying, as they have to specify that HW::Error implements From<X::Error> for all the relevant hardware types X. But, this nuisance is limited to the driver implementations, and doesn't spill over to users of the library.

To keep the boilerplate to a minimum, the type definitions put minimal constraints on their generic types (e.g. Epd2In9 expects a HW and STATE of any type). Each each trait implementation then request just the minimal set of constraints it cares about.

For example, the Reset implementation just requires the hardware constraints HW: ResetHw + DelayHw + ErrorHw and HW::Error: From<<HW::Reset as embedded_hal::digital::ErrorType>::Error>.

To support both displays with different "busy" pin states, and different wiring configurations of the existing displays, the BusyHw trait now also requires a busy_when() function.