spi: interdependent operations/operations sharing data

[jordens]

There is currently no support for SPI operations (in a transaction slice) to mutably share data.

Use case: SPI devices often implement the following behavior (happy to provide numerous examples if this is in doubt) to implement an atomic update of several bits.

• Load "active" bits into the SPI shift register (that sits between SDI and SDO) on CS assertion.
• Load new bits from shift register into "active" register on CS deassertion.

The intention is to implement a read of the active bits without altering them (not even a transient alteration).

The approach would be to keep CS asserted accross a transaction of reading and writing the same values again.

let mut bits = [0; 4];
spi_device.transaction(&mut [Operation::Read(&mut bits), Operation::Write(&bits)]);

This is currently not possible with the Operation enum.