add TraceableValue[F[A, B]] instance for F[_, _]: Bifoldable

[bpholt]

I needed an instance of TraceableValue[Either[A, B]] given A: TraceableValue and B: TraceableValue, and then I realized I could expand that to any F[_, _]: Bifoldable if we're willing to accept the right bias on things like Ior or Tuple2.

(If #1248 is merged, we could use its ListValue to eliminate the right bias, but I don't think it's possible otherwise.)