Add ADC `Voltmeter` and `Ammeter` traits.

[reitermarkus]

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This adds an AdcChannel trait as mentioned in https://github.com/rust-embedded/embedded-hal/issues/377.

Still needs to be tested by implementing an actual driver. I feel there is something still missing with regard to how the return value can actually be used further. Is it even practical to have this trait without having a corresponding unit and a lower/upper bound for converting the raw value?