This makes it uglier for the user, but way easier for library development.

Right now it's hard to parse closures that return waveform generator closures because impl Fn(f64) -> f64 is unsized.

Doing this will unblock a bunch of things, or at least make it way easier:

• Storing instruments/waveform generators in a structure of sorts and passing that around (eg, instrument bank for MIDI)
• Easier type signatures for make_midi_samples, fewer hacks
• Easier time with higher-order closures that return generators