Switch from using ramp to lag for sliding synth args:

* Ramp appears to have an unfortunate delay before kicking in which
  lag doesn't suffer from. This is a shame because linear ramping is
  favourable for some control values.
Fix bug in fn name for joining thread and subthreads in fx block
Use basic sampler synths if no non-modulatable args are used:

* basic sampler synths use less resources
* basic sampler synths support modulatable rate