Implement arpeggiator

Automatically plays notes corresponding to held keys, one at a time, in some order, at some tempo. Possible note order implementations:

- Ascending
- Descending
- Random
- Key press order (requires additional handling of key press order)
- More complicated (e.g. inversions)

For an example of arpeggiation, see the first sound file on this page: https://theproaudiofiles.com/arpeggiators/.