As discussed in today's meeting:

It is better if the CS-assert-to-clock and clock-to-CS-deassert delays are configured in the SpiDevice itself, so that the driver doesn't have to add them to every single transaction, and can use the read/write/write_read convenience methods.

• add configuration settings to the SpiDevice impls in embedded-hal-bus to do the delays
• document that drivers are discouraged from using Operation::DelayNs directly for CS-to-clocks delays.