There is no need to take src_len and dest_len arguments.  The case of
reading-only with a single output byte (originally src_len=1, dest_len>1)
is now handled by using the output buffer as the input buffer, and using
memset to fill the output byte into this buffer.  This simplifies the
implementations of the spi_transfer protocol function.

These symbols are still defined in terms of the system Exxx symbols, and
can be switched to internal numeric definitions at a later stage.

Note that extmod/modlwip still uses many system Exxx symbols.

Supports speeds up to 500k baud, polarity=0/1, phase=0/1, and using any
pins.  Only supports MSB output at the moment.