Allow parametization of DSP graph functions

[harudagondi]

Currently, this could not work:

type SineWave = /* the full type */;

fn sine_wave(freq: f32) -> impl Fn() -> SineWave {
    move || sine_hz(freq) >> split::<U2>() * 0.2
}

All returned closures are of the same type. Thus, they have the same TypeId.

Internally, DspManager uses HashMap<TypeId, DspGraph>. This is built for functions with the type fn() -> impl AudioUnit32. It would be nice if there is a way to generate multiple functions/closures, each with a different type.

Another potential solution is to find the equivalent of Python's is operator. This test if the two objects are the same (not to be confused with equality). A ValueId would be nice, and can be an alternative to Box<dyn Hash> (this, of course, doesn't work).