A foundational type for performable musical instruments.
The Instrument type takes discrete note_on and note_off events as inputs
and returns a Frames iterator yielding a amplitude/frequency value pair per
Voice per Frame as an output.
Instrument supports multiple note handling Modes including n voice
polyphony and n voice unison monophony (both retrigger and legato).
Note-on effects such as detuning and legato are also supported.