Make the nrf Twim RAM buffer a instance variable instead of stack allocated

[alexmoon]

This prevents the ram buffer from being duplicated in every future that calls a Twim transaction method. In my firmware this ended up saving about 4kb of ram.

I can implement this for the other peripherals that implement copying into ram buffers if this looks good to you.

[lulf]

Looks good to me. It's a bit clunky to size this buffer correctly perhaps, is there a way to simplify that?

[alexmoon]

It just needs to be large enough for the largest write operation your firmware sends from flash. I don't think there's any way for embassy-nrf to help with that. We could add a usize to the RAMBufferTooSmall error variant so the error message could say what size buffer was needed? Though that would increase the size of the error enum.

[lulf]

It just needs to be large enough for the largest write operation your firmware sends from flash. I don't think there's any way for embassy-nrf to help with that. We could add a usize to the RAMBufferTooSmall error variant so the error message could say what size buffer was needed? Though that would increase the size of the error enum.

Yeah, I don't like that option either... But maybe at least some guidance in the rustdoc for the new() on the sizing of the buffer?

[lulf]

Thanks!