i/o configuration

[wrl]

right now, i/o is fixed at stereo in, stereo out. this should be configurable somehow.

imo, MVP on this is a fixed channel count defined in the Plugin impl block, eg:

impl Plugin for Gain {
    const NAME: &'static str = "basic gain plug";
    const PRODUCT: &'static str = "basic gain plug";
    const VENDOR: &'static str = "spicy plugins & co";

    const INPUT_CHANNELS: usize = 2;
    const OUTPUT_CHANNELS: usize = 2;
    const MIDI_INPUT: bool = false;

    type Model = GainModel;

vst2 only supports fixed i/o configuration like this, but formats like auv2 have significantly more flexible methods. punting on this for the future.

[wrl]

phase 1 (fixed i/o channel counts) done in 0ff0934