spi: Improve use of PD API version 3

spi: Improve use of PD API version 3

Eliminate more instances where Python code tracked signal changes instead
of having common library code do the work. Reduce the number of boundary
crossings between library and decoder code (Python vs C), by inspecting
fewer input edges. Yet cope with the CS signal's being optional. Emit
identical annotations to the previous implementation for CS changes,
including the initial change "from None to 0/1". Sort the checks for
optional input signals at the top of decode() into a more natural order.
Improve and update comments.

This commit results in a more appropriate use of the PD v3 API in the
SPI protocol decoder.