Return `I: Iterator<&DspNode>` rather than `Vec<&DspNode>`

[mitchmindtree]

Referring to the get_inputs methods, this line in particular. This should still be generic enough so that people can store their DspNode types in unique ways and still easily define the get_inputs method for returning references to them. It should also be a lot faster as it means giving people the ability to write zero-allocation dsp chains! (rather than creating a new Vec for each Node's inputs every buffer).

One issue however would be requiring people to write out the elaborate Iterator return types, however this should be fixed once abstract return types arrives.

[mitchmindtree]

This is no longer relevant to the current API, closing.