Add I2C software_reset method.

[reitermarkus]

Add method for software reset via the “General Call” address.

[rust-highfive]

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[eldruin]

This is an interesting addition which seems fine to me as the traits stand.
However, I wonder how it would look after #392.
Specially there I think there is a non-negligible risk of people thinking it is only going to reset their device and not every device connected (which supports it) since they get access to the bus through an I2cDevice.

device.transaction(|bus| {
    bus.software_reset()
});

I see that software_reset_on_every_device_which_supports_it() seems wonky, though.
I think this would be an interesting point to add to the comments I added here.

PS: I added this to the 1.0.0 milestone only because we need to decide on whether to merge this or not before 1.0.0 due to it being a breaking change.

[eldruin]

This should be added to the unreleased section

[eldruin]

We discussed this in the meeting today and decided against merging it due to a number of reasons:

• It does not seem anybody would need anything else than the default implementation, so it would not build an abstraction that implementations later define, like with the rest of traits.
• It is trivial to do in user code if needed: bus.write(0x0, &[0x06]).
• It has potential for unintended foreign device resets.
• Support for I2C General Call Reset command is rare.

Nevertheless, thank you very much for the proposal!