Refactor props tree

[ericyd]

• Move clock Worker into own component. Place it at the top of the tree and pass it to all children
• Properly use dependency arrays in useEffect hooks
• Move encapsulated functionality into separate components
  • Exporter component handles exporting a track's audio
  • BeatCounter fully encapsulates the process of updating the counter component, so children don't get re-rendered on every beat
• rename worklets to workers; this directory had 3 workers and 1 worklet, it only seemed fair
• use named exports everywhere, rather than defaults (weird preference thing)
• fix bug where track muting wasn't carrying over between loops