The aim here is to have spi.c contain the low-level SPI driver which is
independent (not fully but close) of MicroPython objects and methods, and
the higher-level bindings are separated out to pyb_spi.c and machine_spi.c.
There is an underlying hardware SPI driver (built on top of the STM HAL)
and then on top of this sits the legacy pyb.SPI class as well as the
machine.SPI class. This patch improves the separation between these
layers, in particular decoupling machine.SPI from pyb.SPI.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.