[WIP] i2csensors: Become no_std

[chrysn]

• Numeric traits are replaced with their core counterparts.
• Errors don't need to implement std:error::Error any more.
• Altitude-from-pressure was commented out until there is a libm
  implementation of powf (1, probably)

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This is early work to allow using the I2C sensor traits also on no_std platforms, like in klemens/si7021-rs#1. Before I proceed here, I'd like to solicit the maintainer's opinions:

• Are the type constraints on T::Error to be std::Error actually justified and depended on? (My impression is that as long as one uses base devices whose errors are std, they're usable in the end anyway; only "sensor middleware" like Altimeter for Barometer can't access the error types as std if they're not required, and they can still restrict their implementations to where T::Error: std::error::Error if they do need that.)
• If they need to stay, would you be OK with a configure feature that waives that limitation?

[martindeegan]

Looks good to me. Don't the math operations for Vec3 also need to be removed?

[chrysn]

The math can stay as the traits for Add etc are present in core as well. It'd probably be nice to have it generic over more numeric types as not all microprocessors are good with floats, but first things first.

[martindeegan]

Cool, tell me when this is ready to merge.

Additionally, since I have not had time to maintain this repo, I could add you as a maintainer if you'd like. It would be nice to switch to convert to embedded-hal and be added to their support list.

[chrysn]

Cool, tell me when this is ready to merge.

Will do.

add you as a maintainer if you'd like. It would be nice to switch to convert to embedded-hal and be added to their support list

I probably won't be able to maintain this long-term, but I'd certainly be happy to help with maintaining the crate while it is being adapted to the embedded-hal ecosystem.

[ryan-summers]

@chrysn I recognize that this has been quite a while, but it looks like the referenced issue for num-traits has been closed - is there a powf implementation available now? I'd be happy to take over this PR if you're too busy (I'm also interested in updating the Si7021 device crate to work no-std).