Teach *_synth_default fns to accept lambdas as args:

use_synth_defaults release: 0.1, cutoff: lambda{rrand(30, 120)}

* lambda fns will be called each time a synth is triggered
* lambdas are extracted and stored in a separate thread local to reduce
  runtime computation (rather than searching for them, extracting them
  and calling them on every synth trigger).