Remove nasty metaclass munging for arg default munging:

* Simplifies and optimises implementation
* Super generic behaviour (which the implementation provided) isn’t 
  necessary at this stage
* Just handle basic case of a symbol which references another param
  (which is assumed to not reference another param in turn)
* Move arg normalisation for synth triggers from server to sound.rb so 
  it can be combined with the symbol -> lookup pass over args
* Server’s trigger_synth now assumes all arg vals are numeric
* OSC encoder gracefully handles arg keys which are symbols