implement dasp::signal::Signal on ReadDiskStream

[Be-ing]

I find the API of ReadDiskStream unintuitive:

let ch1 = read_data.read_channel(0);
let ch2 = read_data.read_channel(1);
for f in 0..frames {
  self.buffer[f * 2] = ch1[f];
  self.buffer[f * 2 + 1] = ch2[f];
}

It would be nice to implement dasp::signal::Signal on this type.

[Be-ing]

Related: RustAudio/dasp#168

[BillyDM]

Possibly, I'll think on it. I'm currently busy with other stuff at the moment.