Mix it up

How about we allow people to play more than one note at a time?

```rust
sick_beat!(
hardcore_bass: (220 4, 240 2, 220 4),
noise: (600 3, 590 1, 610 1, 600 3),
actual_music: (…),
);
```

- The names are just for the dev.
- Repeat each section until the longest ends

Start at the `sick_beat!` Rust macro and trace how its note input is parsed and played. Check how the requested named sections and longest-section repetition fit the existing design; done means the example can play concurrent sections with each section repeating until the longest one ends.

Written by the indexing model from the issue text.