Magnetron overhaul

• Deletion: InBuffer and OutBuffer no longer exist. Use the BufferIndex enum instead.
• Change: BufferWriter::read_<n>_and_write is now BufferWriter::read_<n>_write_1
• New: Add BufferWriter::read_<n>_write_2 for stereo signal processing
• Change: spec::Spec is now automation::AutomatableValue
• Change: spec::Creator is now creator::Creator
• Change: Creator::create is now Creator::create_value
• Change: Stage is now stage::Stage
• Change: StageState is now stage::StageActivity
• Deletion: The waveform module has been removed in order to de-specialize waveforms, further changes include:
  • Change: Creator::new no longer accepts the envelopes argument
  • Deletion: Creator::create_envelope has been removed
• Improvement: Magnetron has been overhauled:
  • Change: Magnetron::clear and Magnetron::write have been merged to Magnetron::process
  • Improvement: Magnetron::process evaluates and returns the StageActivity
  • New: Use Magnetron::process_nested to call a Magnetron instance within a Stage
  • Change: Magnetron::mix is now Magnetron::read_buffer
  • Deletion: Magnetron::set_audio_in has been removed
    • This functionality was too specific and it can be easily achieved now by creating a Stage that writes to BufferIndex::External.