Respond to additional input sources besides polling

[mikesol]

Currently, purescript-audio-behaviors only builds the audio graph at a polling rate of ~50Hz. This is unsuitable for reactive audio applications. It would be better if the graph were constructed based on arbitrary input events, which would require two modifications:

• the EngineInfo record would have to include a list of additional event sources; and
• the computation would have to move off of the main thread to a pool of web-workers, as multiple events in rapid succession would clog the rendering pipeline; and
• some sort of incremental function is likely necessary, as even with pre-rendered audio graphs, calculating the graph diff for a complex cluster of events (ie ~20 concurrent MIDI notes) will clog the rendering pipeline.