Allow varied delay duration for `flow`

[nixonyh]

Proposed solution

Use a closure instead of a fixed delay duration:

let f0 = TrackFragment::new();
let f1 = TrackFragment::new();
let delays = [0.1, 0.2]

let f = [f0, f1].ord_flow(|i| delays[i]);
// Or..
flow([f0, f1], |i| delays[i]);

// For constant delays:
flow([f0, f1], |_| 0.5);