blocking DAC trait

[mendelt]

After some discussion on the rust embedded matrix chat during the Oxidize impl days I made a very simple implementation for a blocking DAC trait. This is a blocking implementation to make it usable for implementing in peripherals that use busses that do not have an nb interface like I2C.

I created a sample implementation for the Microchip mcp4725 I2C DAC here; https://github.com/mendelt/mcp4725/blob/dac-hal/src/lib.rs

The trait only supports setting the output of a single DAC channel. I'm planning on trying this out for the AD5668 8 channel DAC using a .split method that returns a channel instance for each of the 8 channels next.

Some discussion. We might want to have prescribed behavior for DACs that use less than the full word-length. The mcp4725 for example uses 12 of the 16 bits. Right now it uses the least significant bits but I'm thinking about fixing this and using the 12 most significant bits. I think this might make more sense in terms of the abstraction the trait provides. Programs using this trait would get more logical behavior when replacing a 16 bit DAC with a 14 or 12 bit one.

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[mendelt]

I experimented a bit more but I'm a stuck on how to implement something like a .split() method for multi channel DACs so they return one struct implementing this trait per channel. I'll need to spend time on other things the coming weeks. So I closed this PR to keep other people from wasting time on it.

[eldruin]

@mendelt have you seen #247 ? Further input would be valuable there.