Support non-blocking serial/SPI/... communication

[dflemstr]

I'm working on embedded-platform and am targeting nrf52840 devices as the first use-case.

Currently, I'm using this code to support serial writes: https://github.com/dflemstr/embedded-platform/blob/f9ae5e7b686fdbf66ee3f172c755aea543d4c102/platforms/nrf52840/src/serial.rs#L19-L23

I haven't started on SPI/I²C/... mappings but I suspect there will be similar challenges in making the code non-blocking.

The execution model I have chosen is to have the main thread always run wfi when there is nothing happening, and rely on interrupts to wake it up, poll a main future, and do some work, before going back to wfi.

Ideally, the baseline behavior I would like to see is to break apart some of the blocking transactions into multiple steps. Let's look at this section for example:

https://github.com/nrf-rs/nrf52-hal/blob/90a8c3c7dbdb478b49580fd2bf2f89bdb9286cc2/nrf52-hal-common/src/uarte.rs#L145-L154

This could either be modeled as an explicit state machine (e.g. an enum) where during each state transition, there is some sort of polling method instead of a while loop. So imagine a SerialTransaction object with a poll() method that would eventually return a successful status.

If this approach sounds sane, I can make it into a PR.