SPI struct implements both SpiBus and SpiDevice

Currently our Spidev wrapper struct implements both SpiBus and SpiDevice from embedded-hal. I think this is likely to lead to some confusion, especially since the SpiBus methods will still actually be using a shared SPI bus, toggling CS, and multiplexing with any other spidev devices on the same bus.

We discussed this a bit in #87 and while the main problem there is resolved by the new embedded-hal design, I think the split suggested is still worth considering. The proposal is to have two wrapper structs, SpidevBus and SpidevDevice perhaps, where:

• SpidevBus implements just the SpiBus trait
  • we configure spidev to not drive the associated CS pin
  • we explain in the documentation that when this struct is used, it must be the only active spidev device on that bus
  • we encourage users to give it a CS pin that is not connected to anything, since whatever pin is assigned to it in the device tree can't be re-used as GPIO (and therefore can't be used as a CS)
  • this struct can then be used with any of the bus-multiplexing crates (like embedded-hal-bus) to generate lots of SpiDevice objects, using generic GPIO for CS control, or used when dedicated bus access is required (eg to generate smart LED waveforms)
  • we are basically lying to the kernel about it being a single device, because there's no way to request exclusive bus access with spidev
• SpidevDevice implements just the SpiDevice trait
  • it's already a perfect fit for SpiDevice, the kernel handles sharing and CS for us, even with other programs
  • it will work just as users expect
  • but their device tree has to be set up to create the device and use the right CS

I'm happy to implement this as a PR if it sounds good to people, but it'd be great to get some feedback first or any suggestions on how it might be better done, especially if there's any clever spidev-related tricks that might be useful.

Start by reviewing the current Spidev wrapper and the embedded-hal SpiBus and SpiDevice implementations, along with the discussion in issue #87. Evaluate splitting the wrapper into SpidevBus and SpidevDevice, including the proposed chip-select and bus-sharing behavior. Done means the API separation and its documentation clearly match the intended kernel and bus-multiplexing semantics.