spi: add settings to embedded-hal-bus SpiDevice impls for CS-to-clock delays.

[Dirbaio]

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.

[Dirbaio]

i'm going to remove this from the 1.0 milestone,

It concerns only embedded-hal-bus, we can do it in 0.1.1 in a backwards-compatible way (like adding .set_blah_delays() methods), or in 0.2 if we don't want to be compatible.