Function adsr

adsr(
    attack_ratio: number,
    decay_ratio: number,
    sustain_level: number,
    release_ratio: number,
): SoundTransformer
Returns an envelope: a function from Sound to Sound. When the adsr envelope is applied to a Sound, it returns a new Sound with its amplitude modified according to parameters The relative amplitude increases from 0 to 1 linearly over the attack proportion, then decreases from 1 to sustain level over the decay proportion, and remains at that level until the release proportion when it decays back to 0.
Parameters
- attack_ratio: number
  proportion of Sound in attack phase
- decay_ratio: number
  proportion of Sound decay phase
- sustain_level: number
  sustain level between 0 and 1
- release_ratio: number
  proportion of Sound in release phase
Returns SoundTransformer
Envelope a function from Sound to Sound
Example
```
adsr(0.2, 0.3, 0.3, 0.1)(sound);
```