[embassy-nrf] Add InputRef, OutputRef, and FlexRef

[alexmoon]

Adds reference types for GPIOs so that they can be borrowed by drivers without being torn down when the driver is dropped. This allows, for example, an Output to be used in a SimplePwm, then the SimplePwm can be dropped and a gpiote::OutputChannel created using the same Output without its configuration being reset (and the pin disconnected) in between.

[lulf]

I don't really like the changes to the API here where you can't just pass the peripheral anymore, but I do understand the need for this. I thought passing &mut p.P0_10 etc. would work for this use case, but seems that no longer the case. Is that due to the Peri changes?

[alexmoon]

Yeah, I agree. I considered making a From<Peri<'_, ...>> impl for OutputRef and InputRef which would use the default level and pull options. That would make it p.P0_10.into() instead of Output::new(p.P0_10, Level::Low).into(). Note that InputChannel and OutputChannel were already taking Input and Output parameters instead of raw peripherals. This is sort of bringing Pwm in line with those.

[0e4ef622]

I'm not sure what the purpose of InputRef/OutputRef are, it seems like adding a reborrow method to Input/Output would suffice.

EDIT: oh if its borrowed it won't drop.

[alexmoon]

EDIT: oh if its borrowed it won't drop.

Yep