Recover good ideas from the worklet branch

[mmontag]

The worklet branch was a disaster, and I don't like the AudioWorklets API. However, I made some other progress that should be recovered:

• All players should write to the same ScriptProcessorNode
• Players should emit events instead of firing callbacks

[mmontag]

Sequencer has been converted to an event emitter. Players now emit player state instead of just true/false.

[mmontag]

Players now write to the same ScriptProcessorNode. #93