Add new_no_miso to Spi FullDuplexMode

[dragonnn]

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Add new_no_mosi function to Spi in FullDuplexMode, this is useful for devices like spi displays with share a single bus. They don't need a Miso connection but do need CS pin for selection with display to write to. Fixes also #791

[bugadani]

At this point, wouldn't it be better to pass in pins as Options?

[dragonnn]

I am not sure how well Options would work with the generic parameters, I suspect making an option function might be harder then it might seem. Also I think that is the last option that make sense, new_no_mosi isn't something you can find in real work from what I can tell.

[bjoernQ]

The bigger problem when using optional pins would be to pass None - we have gpio::NO_PIN for that. It's what we do for half-duplex. However, to have it aligned to other constructors I think it's fine to have this constructor and later maybe add a constructor which works like the half-duplex constructor

[bjoernQ]

LGTM- Thanks