All time math should be based on samples, not TimeInstants

[kirbysayshi]

There are several places where rounding errors make for tricky code and bugs. Ideally:

• all feed calls accept a sampleIndex: number instead of renderTime: TimeInstant
• TimeInstant can still be used for the external api, and when it makes sense to convert from Score values
• Add a SampleTime.fromNanos(nanos:number, hz: number): number conversion utility to avoid instantiating so many TimeInstants.

TimeInstants are really useful for reading values from Scores, but should not be used as the base unit for sample math.