Templates, flexible envelopes

• Improvement: Add templates to Creator.
  • Change: Add generic parameter A: AutomationSpec to Creator.
• Improvement: Introduce WaveformProperties.
  • Change: WaveformProperties replaces WaveformState.
  • New: Add off_velocity property.
  • Improvement: key_pressure property is Option<f64>.
  • Deletion: Remove secs_since_pressed and secs_since_released. Those values are supposed to be aggregated within the render stages.
  • Change: Remove note_suspension argument from Magnetron::write.
• Improvement: Envelopes are modeled as stages and support automations.
  • Change: EnvelopeSpec replaces Envelope.
  • New: Add velocity property. Waveform stages can decide to ignore this property if no velocity sensitivity is desired.
  • New: Add fadeout property. Waveform stages can use any LF source to control the fade-out phase.
• Change: Update Waveform design.
  • Waveform::envelope is of type Stage<T>
  • Stage<T>::render returns StageState::Exhausted when waveform can no longer deliver any non-zero values
  • Waveform exhaustion is tracked in Wavform::is_active