Remove non-blocking SPI code

[natrow]

Code from the current Forward<T> implementation calls non-blocking SPI traits in embedded_hal-0.2.7, potentially returning nb::Error::WouldBlock and then getting caught in an infinite recursion.

Note: I haven't tested it extremely thoroughly but at least reading and writing seem to work. Looking at the implementation in embedded_hal-0.2.7, their transfer function essentially calls the non-blocking .send() and .read() functions until successful, accomplishing essentially what the previous version of this code was trying to do.

Fixes #27

[natrow]

It could be that your hal implementation never returns WouldBlock while using SPI